House and Senate Republicans are at odds over how to allocate war spending, all but guaranteeing a showdown later this year over how to fund defense programs and keep military campaigns going in Afghanistan and Iraq and against the Islamic State.

House Republicans want to dip into the war account to help pay for Pentagon programs they feel would otherwise get squeezed. But their approach will make it necessary for Congress to approve an emergency war funding bill next spring, effectively blowing budget caps — a politically testy issue and a risk Senate Republicans decided not to take.

Instead, the Senate Armed Services Committee produced a policy bill that hews closely to the broader budget deal struck by Democrats and GOP leaders last year, rationalizing that “the agreement is the agreement,” despite leading Republicans’ concerns that they may be shortchanging certain priorities in the process.

At issue is an extra $18 billion worth of war funds that the House Armed Services Committee shifted to Defense Department programs in its annual policy bill, which the House is expected to pass this week.

House Armed Services Committee Chairman Mac Thornberry (R-Tex.) argued that tapping into more war funds is the only way to fund all the administration’s programs and respond to global threats that have worsened since lawmakers agreed on top-line budget numbers last year. Following the House’s plan also means the next president will get to weigh in on defense priorities, Thornberry added, before Congress has to pass any emergency money to keep war efforts going.

“I would argue that I think we’re the ones who are keeping to the agreement,” Thornberry said at a Washington Post editorial board meeting on Monday, acknowledging that “obviously it will have to be reconciled in some way” with the Senate’s bill.

When the Senate defense policy bill comes to the floor next week, Senate Armed Services Committee Chairman John McCain (R-Ariz.) is expected to offer a proposal to increase defense funding, though he has not yet specified how. But the amendment would need the support of Democrats to clear procedural hurdles, and securing those votes will be extremely difficult.

Congress’s Democratic minority opposes the accounting tactic of dipping further into the war account, seeing it as a Republican ploy to funnel more money to pet projects and squeeze more funding for defense without increasing spending for education, infrastructure and other domestic programs. Maintaining such parity is an important priority for the party. Last year, President Obama vetoed a first version of the annual defense policy bill over similar objections.

On Monday, the White House issued a veto threat against the House defense bill for a variety of reasons, including the decision to tap the war funding account, putting Democrats in a familiar bind: support the bill and keep the process going or vote no and risk being accused of leaving soldiers in the lurch.

Almost all House Armed Services Committee Democrats supported this year’s defense policy bill when it was approved by the panel, but such support has often broken when it comes to the floor vote

“At the end of the day, I’m just loath to vote against an authorization for our troops when we have folks in harm’s way right now,” said Rep. Tammy Duckworth (D-Ill.), a veteran who serves on the Armed Services Committee and is running for senator this year. She is one of only 37 Democrats who voted for the annual defense bill over the White House’s objections last year, before Obama vetoed the measure and it was sent back to Congress to be rewritten along the lines of the year-end budget agreement.

“If there’s no significant changes, I don’t support this going forward,” said Rep. Ruben Gallego (D-Ariz.), another veteran on the committee, who voted against last year’s bill. “This is not the way to budget the defense budget, and it’s also a violation of sequestration. I don’t believe you can have it both ways.”

For others, the dilemma is more complicated.

Last year, the House voted on the defense policy bill while congressional leaders were still hashing out a broader budget agreement that included defense spending. That freed up some Democrats, like committee member Rep. Niki Tsongas (D-Mass.) who said she decided not to support the defense policy on the floor during the first vote knowing she would get a chance to back it later once the overall spending deal was reached.

There is no similar parallel budget process at work in Congress this year. But House Democrats see Senate Republicans’ decision not to follow the House’s approach on war funding as “very helpful,” according to a committee aide.

“It means that even if this process continues to go awry here in the House, Democrats could get another opportunity later to fix this and then make a final call on whether they want to support it or not,” according to the same aide.

House Armed Services Committee ranking member Adam Smith (D-Wash.) has still not said how he will vote.

“I know that what the Republicans are hoping is to pass this budget and then do a supplemental in April,” he said after the committee approving the defense bill by a vote of 60 to 2. “We may have to say, look if you’re doing a supplemental, it can’t just be defense.”

Correction: An earlier version of this report had the wrong party affiliation for Rep. Adam Smith.