Debate over a bill that would impose a tentative contract on railway employee unions over the objections of the unions themselves is making for some unlikely bedfellows in the halls of Congress.

A handful of progressive Democrats have predictably sided with the unions, but among the other voices promising to stand up for the rights of workers are Republicans keen to flex their new populist muscles.

A majority of House Republicans voted against imposing the deal. A staunchly conservative lawmaker of Missouri, Senator Hawley, among others, said he would not vote to force rail employees to accept something they voted to reject unless a paid-leave amendment inserted by the House is included in the final bill. 

“Mr. ‘Pro-worker’ Joe Biden wants to use the federal government to force railroad workers in Missouri & around the nation to accept contract terms they rejected. Not with my support,” Mr. Hawley said in a tweet.

Senator Rubio of Florida, another lawmaker with deeply conservative bona fides,  wrote, “If Congress is forced to do it, I will not vote to impose a deal that doesn’t have the support of the rail workers.”

Another unlikely populist in the fight is Senator Cruz. According to the Houston Chronicle, Mr. Cruz told colleagues that they should not be part of the effort to “crush” unions. Standing up for the rail employees, he said, could be good politics.

Such stances among some of the country’s most die-hard Republicans are a far cry from the free-market evangelism of the Republican Party in the 1980s, when President Reagan fired more than 11,000 air traffic controllers after they went on strike. 

The debate over the rail contract is perhaps the most stark evidence yet that the GOP is trying to shed its reputation as a mouthpiece for Chambers of Commerce and morph into a party of blue-collar populists who might appeal to a new working class base of voters. 

It has also put these new blue collar Republicans in sync with an unlikely faction — the progressive fringe of the Democratic Party. Among the nay votes on the bill forcing the unions to back down were a “squad” member, Representative Rashida Tlaib, and a former Congressional Progressive Caucus chairman, Congressman Mark Pocan. “The rail industry must put the quality of life of their employees over profits,” Ms. Tlaib tweeted. 

While the other members of the so-called squad — Representatives Ilhan Omar, Alexandria Ocasio-Cortez, and Ayanna Pressley — all voted to impose the agreement, Democrats from more conservative districts voted to allow the unions to keep negotiating. 

A freshman congresswoman of Alaska, Mary Peltola, and Representative Jared Golden of Maine, both of whom represent districts won by President Trump in 2020, voted against unilaterally enforcing the contract.

“Being pro-union is being pro-Alaska,” Ms. Peltola tweeted after the vote. Mr. Golden said that Congress “undermines the fundamental bargaining power of workers and unions across the country” with the imposition of an agreement. 

That members from more red-leaning districts voted against imposing the deal while Democrats in safe seats voted for illustrates the power of economic populism in these rural areas.

Congressman-elect Chris Deluzio, who will soon represent rural western Pennsylvania, also weighed in on the vote. Mr. Deluzio said he would never “vote to break a strike unless it’s to impose the workers’ terms on greedy corporations.”

Aggressively pro-employee stances have served rural Democrats well. Ms. Peltola and Mr. Golden were just re-elected by respectable margins given that President Trump won both districts in 2020. Mr. Deluzio won his race by more than six points in a district President Biden won by three.

After passing the resolution to enforce the rail contract, liberal Democrats pushed to vote on an amendment that would add seven days of paid sick leave to the existing agreement. That motion was adopted with the support of every Democrat and three Republicans.

The paid leave provision faced a much more difficult path in the Senate. Adopting the seven-day paid leave amendment would have required 60 votes, and Mr. Hawley was the only Republican to publicly come out in favor of it. That provision ended up failing in the Senate on Thursday.  

On Tuesday night after the House passed the paid leave addendum, Senator Sanders of Vermont offered an ultimatum to his newly populist Republican colleagues in the upper chamber: “Put up or shut up.”

“If you can’t vote for this, to give workers today — who really have hard jobs, dangerous jobs — if you can’t guarantee them paid sick leave, don’t tell anybody you stand with working families,” Mr. Sanders said.

The Senate passed the House version of the bill on Thursday afternoon. After the vote concluded, Mr. Cruz shared a fist bump with Mr. Sanders on the Senate floor. “I always knew you were a socialist,” Mr. Sanders chided him. “Nope. I’m not,” Mr. Cruz tweeted shortly afterward. “I just don’t agree with Biden & the Democrats voting to screw the union workers.”