Republicans are preparing to shut down the American government on Friday, in the latest attempt by the party to thwart White House efforts to increase vaccine take-up, by undermining vaccine mandates across the country.

Clamor is growing among some conservatives for Republican senators to oppose a stopgap funding bill, which would fund the government for the next few weeks, unless Democrats agree to not direct money towards enforcing a vaccine mandate for larger companies in the US.

If the disgruntled Republicans, who reportedly include Senator Mike Lee, from Utah, are successful, the government would effectively run out of money on Friday and could be forced to furlough workers and shut down some federal services.

The need for vaccine mandates, which have been introduced by Joe Biden, has taken on additional importance as the US braces for the impact of the Omicron coronavirus variant.

The plot by the right comes after some Republican states have already sought to diminish mandates, by expanding unemployment benefits for employees who have been fired or quit over the requirement to get the vaccine.

On Wednesday, the House Freedom Caucus, a group of rightwing Republicans in the House of Representatives, urged their Senate colleagues to block the funding bill, also known as a continuing resolution, “unless it prohibits funding – in all respects – for the vaccine mandates and enforcement thereof”.

In a letter to Mitch McConnell, the Senate minority leader, the Freedom Caucus said the Democratic-dominated House was set to vote in favor of the stopgap funding bill on Wednesday. The bill will then go to the Senate, where Democrats need Republican votes to pass the bill by Friday night.

The House Freedom Caucus said that deadline gave their Senate colleagues “important leverage” to prevent funding for mandates.

Biden introduced vaccine mandates, which require employees to be vaccinated or submit to weekly testing, for federal workers and contractors in July. In September, Biden ordered healthcare workers to be vaccinated and companies with 100 workers or more to require Covid-19 vaccines or testing, which the government said would cover more than 100 million employees. Those measures have been put on hold by court rulings, after Republican state attorneys general, conservative groups and trade organizations have sued to stop the regulations.

The appeal by House Republicans came after Politico reported that some Republicans in the Senate were open to blocking the stopgap funding bill.

“I’m sure we would all like to simplify the process for resolving the [continuing resolution], but I can’t facilitate that without addressing the vaccine mandates,” Lee told Politico.

“Given that federal courts across the country have raised serious issues with these mandates, it’s not unreasonable for my Democratic colleagues to delay enforcement of the mandates for at least the length of the continuing resolution.”

The Washington Post reported that Lee had planned to at least “block swift debate” on a funding bill.

The growing threat of a funding scrap comes after McConnell said on Tuesday there would be no shutdown. While that came before Wednesday’s push from his colleagues, some Republicans had been clear for weeks that they would use the funding bill to oppose vaccine mandates.

Republican senators threatened a shutdown at the beginning of November, when Roger Marshall, a senator from Kansas, led a group of 11 Republicans who sent a letter to Chuck Schumer, the Senate leader, threatening to stymie funding.

In that letter, signed by 10 Republican senators including Lee and Ted Cruz, Marshall complained that Joe Biden’s plan to require larger businesses to mandate vaccines or weekly testing for workers was “nothing short of immoral”.

“We will oppose all efforts to implement and enforce it with every tool at our disposal, including our votes on spending measures considered by the Senate,” Marshall wrote.

“To be sure, we agree that countless Americans have benefited from the protection offered by the Covid-19 vaccines. Nevertheless, the decision whether to be vaccinated against Covid-19 is a highly personal one that should never be forced upon individuals by the federal government.”

Marshall, a former member of the House who was sworn into the US Senate on 3 January 2021, has promoted a conspiracy theory about federal reports of coronavirus deaths on his Facebook account. Facebook said Marshall’s post violated policies against “spreading harmful misinformation”. Marshall said he was a victim of “corporate censorship”.

Iowa, Tennessee, Florida, and Kansas, which all have Republican legislatures, have changed their rules in recent weeks to expand unemployment benefits for people who have been fired or quit after failing to comply with vaccine mandates. On Wednesday, the Guardian reported that Missouri was contemplating similar laws, with more states likely to follow.