Leaders in the United States Senate have agreed to a stopgap measure designed to prevent an imminent government shutdown over budget legislation.

The deal, hashed out between Republicans and Democrats, would keep the government open for six additional weeks. It would also earmark approximately $6bn in short-term funding for Ukraine as it fends off a Russian invasion.

But should the deal pass the full Senate, it will likely face significant hurdles in the House of Representatives, where far-right Republicans have promised to shoot it down if it reaches the floor.

Seeming to anticipate criticism from the hardline flank of his party, Senate Minority Leader Mitch McConnell defended the short-term solution in a speech on Tuesday.

“Delaying action on short-term government funding doesn’t advance the ball on any policy priorities,” the Republican senator said.

“Shutting the government down over a domestic government dispute doesn’t strengthen anyone’s political position. It just puts important progress on ice and it leaves millions of Americans on edge.”

The US government is less than five days away from its deadline to pass budget legislation at midnight on September 30 (04:00 GMT on Sunday).

Should it fail to do so, much of the government will stop all non-essential functions, leaving government services in limbo and millions of federal employees without their paycheques.

That prospect could also further dent the US’s credit rating, thereby forcing the government to pay higher interest rates.

Already, in August, one of the major credit ratings agencies, Fitch Ratings, downgraded the US’s score from AAA – the highest – to AA+, citing a “steady deterioration in standards of governance”. Analysts with another credit agency, Moody’s, issued a warning on Monday that it could follow suit if the budget standoff is not resolved.

But Republicans, particularly on the far-right, have baulked at current levels of government spending, calling for stiff cuts to expenditures like aid for Ukraine.

“This will certainly lead to a government shutdown,” Republican Senator JD Vance of Ohio said in response to the $6bn for Ukraine in the stopgap proposal.

If the stopgap measure passes the full Senate in the coming days, as is expected, House of Representatives Speaker Kevin McCarthy – the top Republican in the lower chamber – can choose to bring the legislation to the floor.

But he faces a split party, with some in the far-right threatening to remove him from his leadership position if he partners with Democrats to pass budget legislation.

When asked by reporters on Tuesday whether he felt his speakership was on the line, McCarthy issued a punchy response.

“Oh man, that should make me worried, huh?” McCarthy said theatrically. “It’s the same thing you asked me before I got elected speaker. I’m not worried about whether there’s a speakership place or not. The only thing I care about is fighting for the American people.”

In January, it took McCarthy five days and a historic 15 rounds of voting to gain the speakership, amid staunch opposition from the far-right.

But in his remarks, McCarthy embraced some of the priorities espoused by far-right members of his party.

He told reporters he felt aid to Ukraine and the stopgap funding for the government should be “two separate things”. The Republican speaker also called on Democrats to instead invest in more security along the US’s southern border.

“I don’t think Ukraine aid should be taken up in the supplemental,” McCarthy said.

“I don’t quite understand, when you have all these people across the country talking about the challenges happening in America today, that people would go and say, ‘Oh we need to do Ukraine and ignore what’s happening along our border.’ I think that would be the wrong approach.”

Republicans hold a very narrow 221-member majority in the House, out of 435 seats. That means McCarthy would either need to pass bipartisan legislation to avert a government shutdown or rally nearly every member of his party.

In a video statement on Tuesday, Democratic President Joe Biden, a frequent target of conservative complaints over budgeting, accused “a small group of extreme House Republicans” of attempting to shut down the government to further their priorities.

“I’m prepared to do my part but Republicans in the House of Representatives refuse,” he said. “They refuse to stand up to the extremists in their party. So now, everyone in America could be forced to pay the price.”

The Senate recessed on Tuesday after a procedural vote to advance the stopgap deal concluded with overwhelming support: There were 77 votes in favour of starting work on the bill and only 19 against.

“The only solution for avoiding a harmful government shutdown is bipartisanship,” Senate Majority leader Chuck Schumer, a Democrat, said on Tuesday, counting the dwindling number of days until the potential shutdown. “We are now right at the precipice.”