WASHINGTON — Congress brought an end to a three-day government shutdown on Monday as Senate Democrats buckled under pressure to adopt a short-term spending bill to fund government operations without first addressing the fate of young undocumented immigrants.

The House quickly approved the measure — which will fund the government through Feb. 8 and extend funding for the popular Children’s Health Insurance Program for six years — and President Trump signed it on Monday night.

The agreement also revealed fissures among Democrats, with about one-third of the party’s members in the Senate and a majority in the House voting against it.

The passage of the measure ended an ugly, if short-lived, impasse that threatened to give a black eye to both major political parties. The deal, reached after a bipartisan group of senators pushed their leaders to come to terms, enables hundreds of thousands of federal employees who had been facing furloughs to go back to work.

But a key part of the deal, a pledge by Senator Mitch McConnell, Republican of Kentucky and the majority leader, to allow an immigration vote in the coming weeks, sets the stage for a battle over the so-called Dreamers, young undocumented immigrants who were brought to the United States illegally as children.

Lawmakers in the House and Senate are offering drastically different visions of how to resolve their fate. But those on both sides of the debate, as well as advocates for immigrants’ rights, said that ultimately Mr. Trump would need to get involved for the immigration dispute to be settled.

Mr. Trump’s intentions were hard to discern, even as he took time to jab at the Democrats.

“Big win for Republicans as Democrats cave on Shutdown,” the president said on Twitter at 11:30 p.m.

The message continued: “Now I want a big win for everyone, including Republicans, Democrats and DACA,” referring to the Dreamers, “but especially for our Great Military and Border Security. Should be able to get there. See you at the negotiating table!”

The vote in the Senate was lopsided: 81 senators voted to end the shutdown while 18 — two Republicans and the rest of the Democrats and an independent who caucuses with them — sided against the measure. In the House, the vote was 266 to 150, with about three-quarters of Democrats opposed.

The measure also shored up the Children’s Health Insurance Program, known as CHIP, which insures nearly nine million children. States had warned that they were on the verge of having to end coverage after Congress allowed funding for the program, which had been created and sustained for two decades with bipartisan support, to expire in September.

The votes came after a weekend of fevered negotiations by a bipartisan group that eventually grew to include about 25 senators, who helped put together a framework in which Democrats would vote to reopen the government in exchange for the promise from Mr. McConnell.

An apparent turning point came when Mr. McConnell took the Senate floor on Monday morning to announce that he would ensure a “level playing field” on immigration — language that some Democrats interpreted as going further than he had before. Mr. McConnell said he would have the Senate take up immigration legislation by mid-February if the issue had not been resolved by then.

“I sat on the floor and listened to him very intently, somewhat holding my breath,” said Senator Angus King, independent of Maine, who caucuses with the Democrats. “I think the majority leader has made a public commitment that it would be very hard for him not to meet.”

But Senator Kamala Harris, Democrat of California, was unconvinced, and voted against the bill. She suggested that she did not trust Mr. McConnell.

“I refuse to put the lives of nearly 700,000 young people in the hands of someone who has repeatedly gone back on his word,” she said.

Immigrants’ rights activists were crushed.

“Last week, I was moved to tears of joy when Democrats stood up and fought for progressive values and for Dreamers,” said Frank Sharry, the executive director of America’s Voice, an immigrants’ rights group. “Today, I am moved to tears of disappointment and anger that Democrats blinked.”

Hundreds of thousands of young immigrants have been protected from deportation under an Obama-era initiative, Deferred Action for Childhood Arrivals, or DACA. Mr. Trump rescinded the program in September and gave Congress six months, until March 5, to come up with a replacement.

But the president has demanded that border security — including money for the “big, beautiful wall” he has promised at the southern border with Mexico — be included in any package. Mr. Trump also wants limits on what critics call “chain migration,” in which immigrants can sponsor their relatives, and an end to the diversity visa lottery, which fosters immigration from countries that are underrepresented.

A bipartisan group of six senators, led by Richard J. Durbin, Democrat of Illinois, and Lindsey Graham, Republican of South Carolina, has proposed the backbone of an immigration deal that backers hope could garner 60 votes, enough to break a filibuster.

But Mr. Trump has rejected that plan. And the measure is almost certainly a nonstarter in the House, where Speaker Paul D. Ryan has promised a vote on a conservative immigration measure championed by the chairmen of the Judiciary and Homeland Security Committees, if it has the support to pass.

“If we are hoping that Paul Ryan is going to have courage and that the House Republicans are going to be fair and decent and that a bill could emerge, we’re smoking something,” Mr. Sharry said.

Monday’s Senate vote exposed a rift between moderate Democrats who are up for re-election this year in states won by Mr. Trump and their more liberal counterparts.

One of those Trump-state Democrats, Senator Bill Nelson of Florida, characterized the vote as a “big win for the Dreamers,” adding that if the Senate passes a measure with more than 60 votes, it would “put a lot of pressure” on the House to act.

But more liberal Democrats, including Senator Richard Blumenthal of Connecticut, who voted against the spending measure, disagreed.

“The lesson to me is that a promise here is far less meaningful when there is no involvement by the House, not to mention the White House,” he said, adding that he has “no confidence, zero, that Paul Ryan will bring a measure to the floor, in fact on the contrary.”

Mr. Graham said it was critical that the Senate’s eventual immigration bill have the support of a broad bipartisan majority of perhaps 70 senators.

“A partisan product doesn’t get you to where you want to go,” he said. “If you’re going to make the play of trying to pick off a handful of the other side, it’s going to crash and burn.”

The shutdown crisis began Friday, after talks between Senator Chuck Schumer of New York, the Democratic leader, and Mr. Trump to keep the government open broke down when the president and his chief of staff demanded more concessions on immigration.

According to one person familiar with that day’s discussion, Mr. Schumer agreed to more military spending and discussed fully funding the president’s request for a border wall in exchange for an agreement from the president to support legalizing the Dreamers.

Late that night, an overwhelming majority of Democrats, joined by a handful of Republicans, voted to block consideration of a spending bill very much like the measure that passed Monday. The only difference is that the initial bill would have funded the government for four weeks, not three.

A round of partisan finger-pointing ensued, with Democrats calling the impasse the “Trump Shutdown” and Republicans branding it the “Schumer Shutdown.”

At the White House on Monday, Mr. Trump’s press secretary, Sarah Huckabee Sanders, insisted that Monday’s deal was not “drastically different” than what was discussed on Friday between the president and Mr. Schumer.

Despite what was characterized by both parties as Mr. Trump’s invisibility this weekend, Ms. Sanders insisted that he was responsible for making a deal happen.

“What the president did clearly worked,” she said, adding, “The president stayed firm, Republicans stayed firm, and Democrats, I think, realized that they had to move past that piece of legislation” to discuss immigration going forward.

But Ms. Sanders declined to clarify precisely what the parameters of an immigration deal would look like. That worried Democrats like Senator Robert Menendez of New Jersey.

“The problem with all of this is the problem it’s been from the beginning,” Mr. Menendez said. “You got no guarantees from the House. You got no guarantees from the president. So you have two-thirds of the equation that are just not there.”