Treasury Secretary Steve Mnuchin, left, and Charles Rettig, commissioner of the Internal Revenue Service (IRS), listen during an IRS Criminal Investigation 100th year anniversary event at the IRS headquarters in Washington on Monday. (Andrew Harrer/Bloomberg News)

Some of President Trump’s biggest Republican allies in Congress are starting to rebel against the administration’s fallback strategy to avert a government shutdown and a potentially catastrophic debt default this fall.

At issue is a plan put forward by Treasury Secretary Steven Mnuchin and White House acting chief of staff Mick Mulvaney last month: fund federal government operations at current spending levels for one year, paired with a one-year increase in the debt ceiling, which limits the federal government’s borrowing capabilities.

The two administration officials proposed that option in case the White House fails to reach a broader budget and debt limit deal with top congressional officials, mainly House Speaker Nancy Pelosi (D-Calif.).

But in a letter being sent to the Trump administration Wednesday, several GOP senators, including Sen. David Perdue (Ga.), who carries influence with Trump and the White House, are stressing that a spending plan using a one-year “continuing resolution,” or CR, would have a “draconian” impact on the Pentagon and are urging them away from the idea.

“While some members of the administration have suggested a year-long CR as a viable path forward, this must be avoided,” the Senate Republicans wrote in the letter, provided to The Washington Post in advance of its release. “Under these draconian conditions, the DOD would be incapable of increasing readiness, recapitalizing our force, or rationalizing funding to align with the” National Defense Strategy.

The senators note that by using previous years’ spending levels, the administration is, in effect, using priorities for the Pentagon drafted by its predecessors.

“As the world continues to become more dangerous, the American people rightfully expect their representatives in Washington to put aside political differences and do their jobs,” the senators wrote. “Simply put, our adversaries do not handcuff their militaries with funding gimmicks like continuing resolutions — nor should we.”

Those who signed on to the letter include Perdue and 15 other GOP senators, including James M. Inhofe (Okla.), the chairman of the powerful Armed Services Committee; John Cornyn (Tex.), a former member of GOP leadership; Joni Ernst (Iowa) and Thom Tillis (N.C.), two Senate Republicans facing competitive reelection bids next fall; and close administration allies such as Kevin Cramer (N.D.) and Marsha Blackburn (Tenn.).

The letter is addressed to Mnuchin, Mulvaney and Russell T. Vought, the acting director of the Office of Management and Budget.

A senior administration official responded that if the Republicans had a different proposal for “fiscal restraint, then we’re all ears.”

“However, pointing to elusive mandatory spending cuts while letting discretionary spending skyrocket and dismissing it as ‘squabbling’ is a cop-out,” said the official, who spoke on the condition of anonymity to freely discuss internal administration talks. “The letter also ignores the House meltdown over a simple humanitarian supplemental last week, and lacks a plan to stop Democrats from dismantling the president’s deregulatory and border security agendas with poison-pill riders.”

The administration and Congress are barreling toward a potential fiscal pileup later this fall, when on Sept. 30 federal government agencies will run out of money and begin shutting down unless lawmakers and Trump reach a deal to fund them. Around the same time, Washington also needs to raise the debt limit to avert a potential default on its bills — a top priority for Mnuchin, who has been the administration’s point person so far in negotiating with Congress on fiscal issues.

But the brewing opposition from conservative Senate Republicans illustrates the tightrope confronting the administration, as the ultimate plan clearly needs buy-in from Democrats who control the House but must win over enough GOP votes to avert a rebellion from Trump’s own party.

Characterizing it as a “compromise,” Mnuchin and Mulvaney offered the fallback option after a meeting with Pelosi, House Minority Leader Kevin McCarthy (R-Calif.), Senate Majority Leader Mitch McConnell (R-Ky.), Senate Minority Leader Charles E. Schumer (D-N.Y.) and top congressional appropriators last month.

Democratic leaders have declined to rule out that plan explicitly, but instead insist Congress can do better than a fallback option that defaults to previous funding levels and spending priorities. McConnell also indicated in a news conference last week that a one-year continuing resolution, especially from a defense spending perspective, was “unacceptable.”