Senate Republican leaders have warned that the upper chamber might have to remain in session this weekend if it fails to pass a short-term spending bill by Friday.

According to lawmakers who attended a closed-door session Thursday afternoon, Majority Leader Mitch McConnell, R-Ky., told fellow Republicans not to leave town. Assuming the House can pass the bill and send it over, McConnell plans to call up a vote for the spending bill again and again in an effort to pressure reluctant lawmakers, mostly Democrats, to back the bill.

"The leader told us not to leave," Sen. Richard Shelby, R-Ala., said after the meeting.

But a GOP-written spending bill providing government funding until Feb. 16 could be impossible to pass in the Senate, where Democratic opposition is mounting and a few Republicans now say they oppose it.

Some lawmakers are instead calling for a spending bill that is just a few days long to get over the hurdle, although Majority Whip John Cornyn, R-Texas, refused to endorse that option.

McConnell did not offer a multi-day spending bill as a backup plan if the month-long spending legislation does not pass, lawmakers reported after the meeting.

The spending bill needs to win approval in the House first, and it is not entirely certain if Republicans can round up enough support to pass it on their own. Democrats plan to oppose it, citing lack of funding for their wishlist items and a failure to reach a deal on an unrelated matter, the legalization of so-called Dreamers.

Negotiations on separate legislation trading Dreamer legalization for border security continued between Republicans, Democrats, and Trump administration officials in a first-floor office in the Capitol Thursday afternoon.

Democrats were pessimistic about the idea that a deal could be found this week on immigration.

"We've agreed to have additional meetings, but so far, no substance," Sen. Dick Durbin, D-Ill., said.

Durbin called the vote on the spending bill "very close at this point," and described Democrats as "very unified" in opposition.

A House vote is scheduled for 7 p.m.

A large faction of conservatives, the House Freedom Caucus, said about 22 in their group will vote to oppose the measure. Republicans need 216 votes to pass the bill and can afford to lose exactly 22 votes and still pass the measure.