Partisan disputes over the sequester threaten to derail the appropriations process in the months ahead, as House and Senate appropriators trudge ahead with work on fiscal 2016 spending bills.

In the quest for a new budget deal, Democrats in both chambers are vowing to vote en bloc on the floor against any appropriations bills that adhere to the sequester-level spending caps, which were codified into law in 2011. The White House is backing Democrats up, too, threatening to veto any measures that adhere to the statutory framework.

They hope their strategy of stonewalling spending bills, which ultimately served them well during the 2013 government shutdown and the Homeland Security funding standoff earlier this year, will bring GOP leaders to the negotiating table.

It’s a threat that’s already giving some GOP leaders headaches.

“I think we have to,” House Appropriations Chairman Harold Rogers, R-Ky., said earlier this month when asked about negotiating a potential budget deal. “It’s going to have to take a willing White House and Senate-House agreement.” Speaker John A. Boehner, R-Ohio, also suggested recently he is open to such discussions.

Despite that rhetoric, no parties have stepped forward yet to take the lead in convening budget discussions.

Unless or until that occurs, appropriators are effectively left with an unsavory but all too familiar reality: writing bills to spending levels many agree are politically untenable.

“The law is the law, and until the law is changed we don’t really have any option but to mark these bills up in a way the law would allow,” said Roy Blunt, R-Mo., a Senate appropriator and member of the GOP leadership team. “We’ll just have to see how this works out. I don’t think we really know yet.”

Optimism Abounds  . . .  But Roadblocks Ahead

Publicly, at least, GOP appropriators are putting up an optimistic front.

A spokesman for Senate Appropriations Chairman Thad Cochran said the Mississippi Republican plans to begin marking up fiscal 2016 funding measures as soon as next week. The aide said Cochran plans to move all 12 spending bills through committee — something the panel hasn’t accomplished in years — at the statutory spending limit, which is $1.017 trillion for discretionary spending, excluding war-related funding.

Across the Capitol, the House has already passed two fiscal 2016 spending bills that adhere to the same framework, and leaders have scheduled floor consideration of a third measure, Legislative Branch, later this month.

Looking ahead, GOP leaders face steep challenges from several fronts.

As proven by the relatively anemic final vote tallies for the Military Construction-VA and Energy-Water bills passed in the House earlier this month, most Democrats are sticking with the White House to reject what are even typically popular spending bills.

Leaders should not have much trouble advancing the Legislative Branch bill off the floor this month. But it will get significantly tougher from there as the House appropriators begin tackling the nondefense spending bills that are traditionally more contentious.

House GOP leaders must also contend with a contingent of Republican fiscal hawks that typically votes against appropriations measures. Between Democrats and the fiscal conservatives, whips could be hard-pressed to secure 218 votes.

A similar dynamic could be replayed in the Senate, where bipartisan support for spending bills is even more critical. The GOP majority must woo six Democrats to overcome a potential filibuster and advance most legislation through the chamber.

Cochran must also contend with the specific challenge presented by a trio of GOP presidential candidates in the chamber eager to showcase their disdain for Washington business-as-usual.

Rushing Toward August

Timing will also prove to be a factor, particularly in the Senate.

As it is, Senate Appropriations has fewer than 90 days to mark up and debate spending bills before the August recess, when the process naturally loses momentum. Once lawmakers return in September, they must not only contend with a new fiscal year that starts in a matter of weeks, but the need to raise the debt ceiling.

As in past years, the two issues could become coupled by Republicans eager to extract concessions from the Obama White House.

Securing floor time will undoubtedly be a challenge, even though Majority Leader Mitch McConnell, R-Ky., also an appropriator, has vowed to set aside time for spending bills.

The consideration of anything on the Senate floor takes time, particularly under the more open process McConnell has been promoting, and appropriations measures are no exception. Other time-sensitive legislation, including measures to replenish the rapidly depleting Highway Trust Fund and to renew the charter for the expiring Export-Import Bank, could also eat up valuable floor time this summer.

The Senate Appropriations subcommittees have begun drafting their fiscal 2016 spending bills to provisional allocations recently delivered by Cochran.

The Mississippi Republican has offered few hints about how he’s divided the $1.017 trillion in sequester-level discretionary spending between the 12 subcommittees.

But with defense and nondefense caps largely frozen from current enacted levels, funding levels for individual programs are not expected to shift significantly

A major question is how much Democrats choose to engage on the subcommittee and full committee level.

Democratic leader-in-waiting Charles E. Schumer of New York has made it clear that the GOP should not expect help from Democrats on the floor passing spending bills that lock in the sequester.

But Barbara A. Mikulski of Maryland, the ranking Democrat on Senate Appropriations, indicated last week she’s taking a case-by-case strategy when it comes to working on individual spending titles in committee.

“We believe their basic premises are so flawed,” she told CQ Roll Call, adding that Cochran’s 302(b)s will be Democrats’ “first battle” in the committee. “But we’ll go one bill at a time.”