Congress is staring down a potential government shutdown beginning Friday night if it does not pass a temporary government funding bill, as Senate Republicans push for amendment votes that could complicate the formula for passage.

Those amendments include a potential balanced budget amendment backed by Sen. Mike Braun, R-Ind., and an amendment from Sen. Mike Lee, R-Utah, that aims to defund all federal vaccine mandates.

There are also reports from Politico and other outlets that Sen. Marco Rubio, R-Fla., wants to force an amendment vote on a bill he introduced with Sen. Joe Manchin, D-W.Va., this week, to bar federal funding from giving away crack pipes.

“Enough is enough. It’s time to stop the petty tyrants imposing COVID-19 vaccine mandates on families across the country. No child should be denied an education because of his or her personal medical choice,” Sen. Ted Cruz, R-Texas, said of his amendment to bar federal funding from going to schools that mandate coronavirus vaccines for children.

Braun’s office says there could be some Democrat defectors for his amendment – which would be a statute, not a constitutional amendment. It’s functionally similar to a Democrat-sponsored balanced budget amendment the Senate voted on in 2011 and to one proposed by Manchin in 2013.

A government shutdown is widely not expected at this point. But things are still developing.

A source familiar with the continuing resolution talks tells Fox News that it’s not yet clear which amendments there will be votes on or what the threshold for the votes will be. “Those issues are still being ironed out,” the source said.

The source did say to expect “a couple” votes on vaccine mandates, and likely other votes as well.

The threshold will be key.

During the December debate over a continuing resolution Senate Majority Leader Chuck Schumer, D-N.Y., allowed a vote on an anti-vaccine mandate amendment at a simple majority threshold. It failed. But then Democrats were at full strength and Republicans were dealing with some absences. The opposite is the case now.

The Senate regularly comes to deals to placate individual senators while guaranteeing a certain result acceptable to leadership. Cruz forced a vote on a Russia sanctions bill over the Nord Stream 2 pipeline earlier this year, which got more than 50 votes. But Schumer only allowed it to go forward on a 60-vote threshold – so it failed.

Often, “the price of getting your vote is that you lose,” R Street Institute senior fellow for governance James Wallner told Fox News in the wake of the last continuing resolution fight.

But Republicans may have the leverage to force and win simple majority votes this time around. On why the timing is going down to the wire, the source familiar with the matter said, “part of the hold-up is that Ds have an attendance issue, and they’re worried Rs would win some of these votes without full attendance.”

Indeed, Sen. Ben Ray Lujan, D-N.M., is currently recovering from a stroke and is expected to be out for weeks. Sen. Mark Kelly, D-Ariz., is absent as his wife Gabby Giffords deals with appendicitis, and Sen. Dianne Feinstein, D-Calif., is also out. The Senate has no remote voting procedures.

This means that if all Senate Republicans show up to the Capitol for a continuing resolution vote, they can win any amendment vote that is brought to the floor.

This comes as House conservatives are urging Senate Republicans to play hardball and use their leverage to defund any vaccine mandates in the continuing resolution.

“We urge you – and all senators – not just to seek ‘process amendments,’ but to seek the only resolution a free people deserve: defunding President Biden’s harmful vaccine mandates,” the House Freedom Caucus said in a letter this week.

Schumer, meanwhile, is exhorting Republicans to ensure the government stays funded. He says they will be responsible for any shutdown.

“Soon our Republican colleagues must come to an agreement with Democrats for passing a continuing resolution to keep the government open until March 11,” Schumer said Wednesday. “This extension is necessary in order to give appropriators more time to arrive at an omnibus… I dare say Republicans prefer not to have a Republican shutdown.”

Another potentially key dynamic will be if Republicans do win their amendment votes – which remains a major if – whether the House will be able to return to town fast enough to sync up with the upper chamber and avert a shutdown. And if the House eventually has to vote on an altered funding bill, it’s unclear whether the Democrat majority will support it if it contains anti-vaccine mandate provisions.

House Speaker Nancy Pelosi, D-Calif., said during the last funding fight the House would not entertain Senate Republicans “anti-vaxxing.” Her office did not immediately respond to a request for comment Thursday asking whether that remains the case.