President Trump's last-ditch effort to secure another enormous package of emergency coronavirus relief is being threatened by an unusual group: his GOP allies in Congress.

For almost four years, Republican leaders have rallied behind the president on issues as varied as health care, immigration, trade and defense, even when his positions bucked long-held conservative doctrines.

Yet just weeks before the Nov. 3 election, as the embattled president is exhorting Congress to move a major package of COVID-19 aid, those same lawmakers have emerged as the single greatest barrier standing in his way.

Senate Majority Leader Mitch McConnell (R-Ky.) has repeatedly thrown cold water on the idea of spending trillions more dollars to fight the pandemic, citing the opposition of roughly 20 Republicans in the upper chamber warning of the long-term effects on the federal debt.

On a conference call Saturday, Senate Republicans relayed those concerns to party leaders and top Trump officials, trumpeting their objections to a $1.8 trillion package proposed by the White House a day earlier and ensuring that the only path to passage is on the shoulders of Democratic votes — a strategy McConnell has rejected virtually out of hand.

The resistance comes at a crucial point in the presidential campaign, when Trump is recovering from his own bout with COVID-19, trailing badly in the polls and all but pleading with Republican leaders to “go big” — and throw him a late-cycle lifeline to promote on the trail.

“I would like to see a bigger stimulus package, frankly, than either the Democrats or the Republicans are offering,” Trump told Rush Limbaugh, a conservative radio host, shortly before Treasury Secretary Steven Mnuchin delivered the $1.8 trillion offer to Speaker Nancy Pelosi (D-Calif.).

Democrats have rejected the White House proposal, saying it simply doesn’t provide enough funding to address the dual crises of health and economy sparked by the pandemic, particularly when it comes to ensuring fast and reliable testing. They’ve held firm to their own $2.2 trillion package, which passed through the House earlier in the month. That amount is already a step down from the $3.4 trillion measure House Democrats passed in May.

Yet administration negotiators have been forced to confront not only the demands of Democrats, but also defiant lawmakers in their own party, who’ve shown no appetite for another massive spending bill and, with Trump down in the polls, have been increasingly willing to push back against the wishes of their ally in the Oval Office.

On Saturday’s conference call, Sen. Marsha Blackburn (R-Tenn.) characterized the $1.8 trillion proposal as a “death knell” for Republicans seeking to promote themselves as the party of fiscal responsibility.

“I liked the package that we had on the floor,” Blackburn told reporters Monday in the Capitol, referring to a much smaller proposal, providing roughly $300 billion in new stimulus, that received a vote last month. “It was the right approach.”

Larry Kudlow, Trump’s top economic adviser, is downplaying the internal divisions, noting that Senate Republicans mustered 52 GOP votes for that smaller bill, with only one Republican defector.

“They united,” he said Sunday on CNN’s “State of the Union” program. “I think, if an agreement can be reached, they will go along with it.”

White House press secretary Kayleigh McEnany delivered a similar prediction Monday, saying the impasse is not the result of Republicans spooked by another huge spending package, but Democrats’ unwillingness to budge from their $2.2 trillion demand.

“I believe Senate Republicans would ultimately come along with what the president wants," McEnany said in an interview with “Fox & Friends.”

Pelosi, meanwhile, is facing her own pressure to seal a deal that can reach Trump’s desk before the elections. And it’s not only moderate Democrats and Republicans — those facing the toughest reelections next month — who are urging the Speaker to accept the latest offer from Mnuchin. A leading progressive voice in the House also took to Twitter over the weekend to press Pelosi to take the deal and pass the package through the House, saying it would put all the focus on McConnell and Senate Republicans.

“People in need can’t wait until February,” said Rep. Ro Khanna (D-Calif.), who like Pelosi represents a liberal San Francisco Bay Area district.

“1.8 trillion is significant & more than twice Obama stimulus. It will allow Biden to start with infrastructure. Obama won in 08 by doing the right thing on TARP instead of what was expedient,” he added, referring to the $700 billion Troubled Asset Relief Program passed by Congress in October 2008.

“Make a deal & put the ball in McConnell court,” Khanna said.

Some Senate Republicans on Monday said the White House still hadn’t shared details of Trump’s new proposal, even as specific dollar amounts continued to trickle out to the press. Their comments highlighted the fact that it's the size of the White House package, not the specific policy prescriptions, that's given Republicans pause.

The $1.8 trillion plan calls for $300 billion for cash-strapped cities and states, up from $250 billion in a previous GOP offer, sources familiar with the package said. And in addition to $1,200 stimulus checks for most Americans, families would receive $1,000 per child rather than tax credits.

The GOP package, sources said, also includes $400 per week in extra unemployment benefits through Jan. 1, retroactive to Sept. 12; $300 billion to replenish the Paycheck Protection Program for small businesses; $175 billion for testing, contact tracing and vaccines; $150 billion for schools; $60 billion for rental and mortgage assistance; $40 billion for airlines and lodging industries; $25 billion for student loan forgiveness; another $25 billion for child care; $15 billion for food stamps and other food security programs; and $10 billion for the Postal Service.

“I haven’t seen the White House package … We were not told what’s in the package. We talked about an amount,” Sen. John Kennedy (R-La.) told reporters during a break from the Supreme Court confirmation hearing Monday. “The only thing we have to base any kind of assertions on in terms of what’s in the so-called package is reporting by the commentariat.”

It remains unclear when, or if, the sides can reach an agreement with just three weeks remaining before the election. And complicating the debate, Trump has vacillated between urging a massive new spending package, shutting down the talks altogether and promoting a piecemeal approach that falls somewhere in between.

Kudlow on Sunday endorsed the targeted strategy, pushing the sides to get behind a handful of provisions that both parties support, including unemployment insurance, small business loans and direct checks to families and individuals.

“If we could get this thing settled on the Democrat side, we will get it settled on the Republican side,” Kudlow said.

Jordain Carney contributed.