House GOP leaders' plan to avert a partial government shutdown at the end of this month could be derailed by mounting opposition from fiscal hawks within their own party.

Speaker Mike Johnson, R-La., rolled out legislation late last week to extend the current year's government funding levels through March via a continuing resolution (CR) to give congressional negotiators more time to work out the next fiscal year's spending priorities.

It's attached to a Republican-led bill for a proof of citizenship requirement in the voter registration process.

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Despite former President Donald Trump blessing the plan, Johnson can afford little room for error with a razor-thin House majority of just four votes.

As of early Monday evening, it appears that at least four House Republicans are opposed to their leaders' bill – one more GOP "no" vote would ensure Johnson will need to seek out Democratic support for its passage.

Rep. Cory Mills, R-Fla., told reporters on Monday that he and Reps. Tim Burchett, R-Tenn., and Thomas Massie, R-Ky., are all opposed.

"I've made it clear…that I'll be a no on the CR," Mills said. "As far as I'm concerned, this is nothing more than messaging."

Fox News Digital reached out to Burchett to confirm his stance.

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Massie told Fox News Digital last week that he believed it was a mistake for Johnson not to push for a longer CR. Under a bipartisan deal passed last year, a CR extending past April 30 would automatically trigger a 1% government funding cut.

"Speaker Johnson has this teed up in front of him. The 1% cut is in law. All we need is a one-year CR to queue it up. When the April 30 deadline arrives, he could even trade the cut for something. But he’s afraid to even create a spending cut deadline," Massie said.

Rep. Matt Rosendale, R-Mont., who is retiring at the end of this year, also told Fox News Digital last week that he is against the bill.

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Lawmakers anticipate a Wednesday vote on the plan, but enough opposition could force House GOP leaders to scuttle the vote.

It's possible that some Democrats will vote for the bill. Five House Democrats voted with Republicans to pass the Safeguarding American Voter Eligibility (SAVE) Act earlier this year.

But Democratic leaders in the House and Senate have both come out in opposition to the plan. The White House also announced Monday that President Biden would veto the bill if it got to his desk.

Johnson told reporters earlier in the day that he did not have a fallback plan in case of failure.

"There is no fallback position. This is a righteous fight. This is what the American people demand and deserve," Johnson said.

It's a position that is likely to worry moderates who worry the political fallout from a government shutdown weeks before Election Day could cost them their seats.

"If we shut down, we lose," one Republican told Fox News Digital last week.

Fox News Digital reached out to Johnson for comment on the "no" votes.

Fox News' Tyler Olson contributed to this report.