WASHINGTON ­— Senate Armed Services Committee leaders John McCain and Jack Reed are asking Defense Secretary Jim Mattis what it means to the military if Congress cannot pass a budget on time this year.

When Congress returns from recess on Sept. 5, it will restart the clock to the Sept. 30 deadline for lawmakers to pass a 2018 federal budget. Lawmakers are expected to follow years of recent precedent and pass a stop-gap budget measure, known as a continuing resolution or CR, to fund the government at the level of the previous year and prevent new start programs unless exceptions are made.

Comptroller David Norquist told Defense News in an Aug. 23 interview that he “wouldn’t be surprised” if Congress opts for a CR at the end of September, when the fiscal year ends.

On Wednesday, McCain, R-Ariz., and Reed, D-R.I., the chairman and ranking member of the SASC, respectfully, made public an Aug. 29 letter they sent Mattis asking him to tell them by Sept. 8 what three- and six-month CRs would mean to the military. They mean to press congressional leaders to start budget negotiations that will ease statutory budget caps for defense and nondefense spending.

“It is long past time for Congress and the White House to begin serious negotiations on a budget deal,” McCain said in a statement. “We know from previous testimony before the Senate Armed Services Committee that continuing resolutions have significant negative impacts on our military. If this is the path Congress decides to take, we must know the consequences we are choosing for our men and women in uniform.”

The two lawmakers warn that failing to reach a budget agreement will amount to billions in cuts to the defense budget, which the Pentagon “can ill afford at a time of diminished readiness, strained modernization, and increasing operations.”

“As the Senate prepares to consider a continuing resolution for fiscal year 2018, we believe it would be prudent to have a concrete understanding of its impacts on the military,” the letter reads. “Therefore, we ask that you provide the committee members with an in-depth list of impacts on the military — including military branches, defense-wide agencies, and combatant commands ­— for a 3-month and a 6-month continuing resolution.”

The timing of the government’s budget has been delayed all year. The Trump administration delivered its budget to Congress months later than regular order would entail, and Congress then spent most of the summer embroiled in the fight over health care reform before exiting for the August recess.

When lawmakers return in September, they will have to work out not just the budget fight but also the question of raising the debt ceiling and deal with push from the White House for new infrastructure and tax legislation — all of which combines to leave the expectation for many that Congress will put together a short-term CR and kick the can down the road for weeks, if not months.

Earlier this month, Mattis called the idea of a CR ”about as unwise as can be” and pledged that he would be lobbying Capitol Hill to try and avoid such a situation. 

If Congress fails to act, or U.S. President Donald Trump blocks that action, it could result in a government shutdown. Congress watchers, however, view that as an unlikely prospect.

The last extended government shutdown, in October 2013, resulted in unpaid furloughs for civilian workers at the departments of Defense and Veterans Affairs.

Service members’ duties continued uninterrupted, but some military pay and benefits were delayed during the 18-day shutdown. In addition, services like daycare and commissary operations on military installations were disrupted or cut back.

Aaron Mehta in contributed to this report.