President Obama’s plan to train and equip moderate Syrian rebels exposed a deep rift Thursday among Democrats over waging war, with a large bloc of liberals staunchly opposed to the modest mission, fearing another long-term engagement in Iraq.

While the Senate sent the measure Thursday to the White House for Obama’s signature, votes this week demonstrated the tenuous support he has from his own party in carrying out the mission to “degrade and destroy” Islamic State forces. Several of the party’s rising stars, including Sens. Elizabeth Warren of Massachusetts and Kirsten Gillibrand of New York, rejected the proposal, while in the House, Obama’s proposal won approval only because a vast majority of Republicans backed him.

Many rank-and-file Democrats who did support Obama said they expect a broad debate in November and December, after the midterm elections, so that legislation can be approved to place broad constraints on the U.S. military’s ability to carry out the operation and set a specific deadline for the mission’s end.

After the votes, Obama thanked Congress “for the speed and seriousness with which they approached this issue” and noted that “a majority of Democrats and Republicans in the House and the Senate” had voted to train and equip the rebels.

“We are strongest as a nation when the president and Congress work together,” he said.

Here's how Congress voted on President Obama’s plan to train and equip moderate Syrian rebels.

Senators voted 78 to 22 to approve the strategy as part of a measure meant to keep the federal government operating through mid-December. Support came from 45 Democrats and 33 Republicans, while 10 members of the Senate Democratic caucus and 12 Republicans voted against the bill.

The vote was not a pure reflection of support for Obama’s plan. Some Republicans opposed it because they felt the bill sets federal spending too high. Others in both parties were opposed because of concerns about plans to vet potential Syrian rebel groups.

Warren said in a statement that she is “not convinced that the current proposal to train and equip Syrian forces adequately advances our interests.” She added: “I do not want American to be dragged into another ground war in the Middle East.”

While Gillibrand said in a statement that she supports “aspects” of Obama’s plans, she said that “previous history leads me to conclude that arming Syrian rebels would be an ineffective solution.”

Sen. Rand Paul (R-Ky.), a potential presidential candidate, voted no and used a combative floor speech to blast Obama and congressional colleagues for not holding a fuller war debate before the elections.

“The people in this body are petrified — not of ISIS — but of the American voter,” he said, using an acronym for another name for the Islamic State. Lawmakers are “afraid to come forward and vote on war now.”

Among other senators who say they’re mulling presidential campaigns, Sens. Ted Cruz (R-Tex.) and Bernard Sanders (I-Vt.), who caucuses with Democrats, voted no; Sen. Marco Rubio (R-Fla.) supported the bill.

The Senate adjourned Thursday night, and the House also is expected to adjourn, capping a two-week congressional session held primarily to ensure that the federal government won’t shut down when the new fiscal year begins Oct. 1. Neither chamber is scheduled to reconvene until after Election Day.

Despite clamoring from the Democratic rank-and-file for a broader debate later in the fall, Senate Majority Leader Harry M. Reid (D-Nev.) and House Minority Leader Nancy Pelosi (D-Calif.) have been counseling patience.

Reid demurred when asked Thursday how expansive the post-election debate should be. He cited it along with a host of other issues, ranging from allowing states to impose an Internet sales tax to confirming more presidential nominees. “We have a lot to do in the lame-duck,” Reid said.

He added that the National Defense Authorization Act, which he also expects to debate after the election, has language related to Syrian rebels. “It’s already in the bill,” he told reporters, suggesting a broad debate might not be necessary.

But currently that language is a small piece of a larger bill that, if enacted, would merely ratify the modest plan to train and arm rebels that Congress approved this week.

Reid’s response mirrored Pelosi’s belief that, under Obama’s current war plans, there is no need for an expansive debate. “It depends on what the president does. We believe that he has the authority to do what he is doing now,” she told reporters Thursday.

But Sen. Richard J. Durbin (D-Ill.), the No. 2 Senate Democrat, said Thursday that a full war debate would take place in the lame-duck session leading to “one of the most important votes we can cast.”

“It’s long overdue,” said Durbin, citing the 2001 and 2002 war authorizations as outdated for today’s threats. “We are living on borrowed time, and we’re traveling on vapors.”

But some Democrats opposed to deeper military intervention said their colleagues had been duped into believing a big war debate was coming. “That’s the illusion,” said Rep. Peter Welch (Vt.), one of 85 House Democrats who opposed the president’s request. “This was the vote.”

Welch, like many other Democrats, is skeptical that the United States will ever be able to identify trustworthy partners on the ground in Syria. “You’re going to have a bunch of white guys vetting these people over in Syria. Our confidence abounds, but that’s tough,” he said.

At least some debate must occur later this year because the current war authorization is to sunset when the short-term spending expires. Several House Republicans who voted against Obama’s new strategy expressed an eagerness Thursday to revisit the issue.

Rep. Raúl R. Labrador (R-Idaho) — an outspoken leader in the conservative wing of the House GOP — said that Republicans will need to be cautious and deliberate. “We used to tread very carefully when it came to the use of force,” he said. “That’s what being a Republican used to be.”

Rep. Cynthia M. Lummis (R-Wyo.) said she grappled with her decision to vote no and is willing to reconsider after a longer debate.

“It is going to be a profoundly difficult decision that I intend to be perched on from now until Dec. 11,” when the current authority expires, she said.