President Donald Trump is blocking a measure to give back pay to federal contractors affected by last month's government shutdown as part of a bipartisan agreement to avert another federal closure, a Republican lawmaker says.

“I’ve been told the president won’t sign that,” Sen. Roy Blunt, R-Mo., told reporters Wednesday. He added, “I guess federal contractors are different in his view than federal employees."

Hundreds of thousands of federal workers were furloughed or required to work without pay during the spending impasse that led to the 35-day partial shutdown. In past shutdowns, Congress has agreed to provide back pay to government workers, but that generally has not extended to federal contractors.

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Democratic leaders in the House and Senate are pushing the provision, which has the support of many Republicans. Lawmakers argue there should be some relief for contractors affected by the shutdown, which was the longest in U.S. history. A number of Republicans have pointed to precedent, however, saying they have never given back pay to contractors after previous shutdowns.

“I understand it’s somewhat complicated, but I think it’s on the negotiators' radar screen, and hopefully they’ll figure a way to work through it,” Senate Majority Whip John Thune, R-S.D., said. “I just don’t think it’s ever been done before, so figuring out duration of contracts — it sounds like it’s the timing issues and all that.”

Democratic Sen. Chris Van Hollen of Maryland and Republican Sen. John Cornyn of Texas illustrated the divide among lawmakers on the issue in a Twitter spat Wednesday.

"I certainly hope this isn’t true," Van Hollen tweeted. "It would be cruel and unnecessary to block back pay for federal contract workers who lost more than a month of wages and are still behind on bills due to President Trump’s shutdown. Many of them work low-wage jobs and live paycheck-to-paycheck."

I certainly hope this isn’t true. It would be cruel and unnecessary to block back pay for federal contract workers who lost more than a month of wages and are still behind on bills due to President Trump’s shutdown. Many of them work low-wage jobs and live paycheck-to-paycheck. https://t.co/sYcKmvkg5Z — Chris Van Hollen (@ChrisVanHollen) February 13, 2019

Cornyn shot back: "Ms. Pelosi and Sen. Schumer should have thought about this and other collateral damage when they initially refused to negotiate on border security, something they are apparently now willing to do."

Trump is likely to sign the bipartisan deal to keep the government open and avoid another partial shutdown at the end of the week “barring any drafting surprises or last-minute additions by Democrats,” officials familiar with the president’s thinking told NBC News on Wednesday. The deal, the details of which first emerged on Monday, does not include the president's sought-after $5.7 billion in border wall funding, which triggered the last shutdown.

Sen. Lisa Murkowski, R-Alaska, told reporters that she would like to see the measure included to provide relief to contractors, and didn't know why the president wouldn't want to include it.

House Majority Leader Steny Hoyer, D-Md., told reporters on Wednesday that “35 days without pay is a really long time to go without pay for people living paycheck to paycheck.”

“We are working on that," he said. "Whether or not we accomplish that — I hope we will, but that has not been done at this point."