WASHINGTON — President Trump doubled down on one of the biggest gambles of his presidency on Tuesday night with a televised appeal to pressure Congress into paying for his long-promised border wall, even at the cost of leaving the government partly closed until lawmakers give in.

Embarking on a strategy that he himself privately disparaged as unlikely to work, Mr. Trump devoted the first prime-time Oval Office address of his presidency to his proposed barrier in hopes of enlisting public support in an ideological and political conflict that has shut the doors of many federal agencies for 18 days.

In a nine-minute speech that made no new arguments but included multiple misleading assertions, the president sought to recast the situation at the Mexican border as a “humanitarian crisis” and opted against declaring a national emergency to bypass Congress, which he had threatened to do, at least for now. But he excoriated Democrats for blocking the wall, accusing them of hypocrisy and exposing the country to criminal immigrants.

“How much more American blood must we shed before Congress does its job?” Mr. Trump asked, citing a litany of grisly crimes said to be committed by illegal immigrants. Asking Americans to call their lawmakers, he added: “This is a choice between right and wrong, justice and injustice. This is about whether we fulfill our sacred duty to the American citizens we serve.”

Democrats dismissed his talk of crisis as overstated cynicism and, with polls showing Mr. Trump bearing more of the blame since the partial shutdown began last month, betrayed no signs of giving in. The White House earlier in the day dispatched Vice President Mike Pence and others to Capitol Hill to try to shore up Senate Republicans, who are growing increasingly anxious as the standoff drags on.

In their own televised response on Tuesday night, Speaker Nancy Pelosi of California and Senator Chuck Schumer of New York, the Democratic leader, accused the president of stoking fear and mocked him for asking taxpayers to foot the bill for a wall he had long said Mexico would pay for.

“President Trump must stop holding the American people hostage, must stop manufacturing a crisis and must reopen the government,” Ms. Pelosi said.

In taking his argument to a national television audience and on a trip to the Texas border he plans to take on Thursday, Mr. Trump hoped to reframe the debate. After spending much of the first two weeks of the shutdown cloistered in the White House, he has now opted to use the powers of the presidency to focus public attention on his ominous warnings about the border.

Yet privately, Mr. Trump dismissed his own new strategy as pointless. In an off-the-record lunch with television anchors hours before the address, he made clear in blunt terms that he was not inclined to give the speech or go to Texas, but was talked into it by advisers, according to two people briefed on the discussion who asked not to be identified sharing details.

“It’s not going to change a damn thing, but I’m still doing it,” Mr. Trump said of the border visit, according to one of the people, who was in the room. The trip was merely a photo opportunity, he said. “But,” he added, gesturing at his communications aides Bill Shine, Sarah Huckabee Sanders and Kellyanne Conway, “these people behind you say it’s worth it.”

Mr. Trump plans to head to the Capitol on Wednesday to attend a Senate Republican lunch and later will host congressional leaders from both parties to resume negotiations that so far have made little progress. Mr. Trump has insisted on $5.7 billion for the wall, while Ms. Pelosi said she would not give him a dollar for a wall she has called “immoral.”

In a nod to Democrats, Mr. Trump spent the first half of his talk on the humanitarian situation at the border before even mentioning the wall, expressing sympathy for those victimized by human smugglers. “This is a humanitarian crisis — a crisis of the heart and a crisis of the soul,” he said.

Even so, he directly took on Ms. Pelosi and Mr. Schumer. “The only thing that is immoral is the politicians to do nothing and continue to allow more innocent people to be so horribly victimized,” he said.

Mr. Trump has made the wall the singular focus of his presidency as he enters his third year in office. His promise to erect a “big, beautiful wall” along the border became perhaps the most memorable promise on the campaign trail this fall, eliciting chants from supporters of “build the wall,” and he has been frustrated by his inability to deliver on it.

But his alarming description of a “crisis” at the border has raised credibility questions. While experts agree there are serious problems to address, migrant border crossings have actually been declining for nearly two decades. The majority of heroin enters the United States through legal ports of entry, not through open areas of the border. And the State Department said in a recent report that there was “no credible evidence” that terrorist groups had sent operatives to enter the United States through Mexico.

At one point in his speech, he even suggested that Democrats had signaled that they would accept his wall if redesigned. “At the request of Democrats, it will be a steel barrier rather than a concrete wall,” he said, even though Democratic leaders have made clear they oppose the barrier regardless of the material.

Even so, Democrats, many of whom like Mr. Schumer voted in 2006 for 700 miles of fencing along the border, did not want to conduct the debate on Mr. Trump’s terms. Instead, they focused attention on the damaging effects of the shutdown, already the second longest in American history. About 800,000 government employees are either furloughed or working without pay, in addition to hundreds of thousands of contractors.

House Democrats planned to approve individual spending bills this week that were intended to reopen closed departments one at a time in hopes of putting Republicans on the defensive, but Senator Mitch McConnell, Republican of Kentucky and the majority leader, has said he would not put any bill on the floor without Mr. Trump’s explicit support.

Senate Democrats took to the floor on Tuesday to pressure Mr. McConnell and vowed to block consideration of other legislation until the government is reopened.

Mr. McConnell fired back, noting the 2006 legislation. “Maybe the Democratic Party was for secure borders before they were against them,” he said. “Or maybe they’re just making it up as they go along. Or maybe they are that dead-set on opposing this particular president on any issue, for any reason, just for the sake of opposing him.”

But two more Republicans, Senators Lisa Murkowski of Alaska and Shelley Moore Capito of West Virginia, spoke out on Tuesday in favor of reopening the government while negotiations over border security continue. “I think we can walk and chew gum,” Ms. Murkowski told reporters.

Ms. Capito expressed frustration with the shutdown and “how useless it is,” indicating that she might support reopening the government while wall talks continue. “I mean, I think I could live with that, but let’s see what he says tonight,” she said before the speech.

That makes five Republican senators who have expressed such a position, which if combined with a unanimous Democratic caucus would make a majority to reopen the government if Mr. McConnell were to allow a vote.

Allies of the president warned fellow Republicans to stand with Mr. Trump. “If we undercut the president, that’s the end of his presidency and the end of our party,” Senator Lindsey Graham, Republican of South Carolina, said on Fox News after the speech.

The political nature of the fight was hard to miss. Just hours before he went on air, Mr. Trump’s re-election campaign sent out a fund-raising email asking supporters to raise $500,000 by the time his speech began. On the other side, Senator Bernie Sanders of Vermont, a possible candidate for the Democratic presidential nomination next year, offered his own response after Ms. Pelosi and Mr. Schumer.

For all of the pyrotechnics of competing national speeches, it seemed like a political Kabuki dance that by the end of the evening had changed no minds in Washington.

Representative Hakeem Jeffries of New York, the No. 5 House Democrat, who perhaps most succinctly summed up his party’s response: “We are not paying a $5 billion ransom note for your medieval border wall,” he wrote on Twitter, with a castle emoji. “And nothing you just said will change that cold, hard reality.”

If Democrats do not approve money for the wall, Mr. Trump has threatened to declare a national emergency and proceed with construction without Congress, a move that could provoke a constitutional clash with the legislative branch over the power of the federal purse. While some legal experts said the president has a plausible case given current law, it would almost surely generate a court challenge.

Even some Republicans warned against it. Senator Susan Collins of Maine said that although the law provides the president with emergency powers, “the administration should not act on a claim of dubious constitutional authority.” She added, “It should get authorization from Congress before repurposing such a significant sum of money for a border wall.”

The wall is popular with Mr. Trump’s base, but the public at large holds the president responsible for the shutdown, according to polls. In a Reuters-Ipsos poll, 51 percent of respondents said that Mr. Trump “deserves most of the blame,” up four percentage points from earlier in the crisis, while 32 percent pointed the finger at congressional Democrats.

Moreover, the public seems to have grown weary of the impasse. Seventy percent of registered voters in the latest The Hill-HarrisX poll favored a compromise, while just 30 percent said sticking to principles was more important than reopening the government.

The president’s use of the Oval Office for the speech stirred some debate, with critics asserting that a setting more typically used for occasions of war or other national security crises was being turned into a partisan platform. The subsequent joint statement by Ms. Pelosi and Mr. Schumer was the first time opposition leaders were given national airtime to respond to a president in the Oval Office.

Not counting speeches to Congress, Mr. Trump had made only five formal addresses to the nation before Tuesday night, three of them in prime time and none from the Oval Office, according to Mark Knoller, a longtime CBS News journalist who tracks recent presidential history. Mr. Trump’s previous prime-time speeches were to introduce his two Supreme Court nominations and to announce his decision to send more troops to Afghanistan.

By contrast, President Bill Clinton gave 16 addresses to the nation over eight years, 14 of them from the Oval Office. President George W. Bush gave 23 such addresses, six from the Oval Office, and President Barack Obama gave 12, with three from his office.

Reporting was contributed by Maggie Haberman, Julie Hirschfeld Davis, Sheryl Gay Stolberg, Michael Tackett, Emily Cochrane and Catie Edmondson from Washington, and Michael M. Grynbaum from New York.