Congress returns Tuesday after a seven-week summer break with one major task to accomplish: Don’t shut down the government.

Lawmakers have four weeks to hammer out a spending deal before the fiscal year ends Sept. 30, but some are already warning the task will not be easy.

The tension surrounds how long a stopgap funding bill keeping the government open, known as a continuing resolution (CR), should last.

Conservatives want to pass a six-month CR that would kick the larger budget fight into next year with a new president and Congress in place. They argue that coming back after the election to complete the annual budget work during a “lame-duck” session will result in a massive legislative package that hikes spending and contains policies sought by special interests.

But Democrats and some moderate Republicans want to finish the annual budget work later this year after the election.

Both sides are digging in for a fight.

“We are not going to agree to a long-term [continuing resolution],” Senate Minority Leader Harry M. Reid (D-Nev.) told reporters Thursday — his comments coming with the implicit threat of a filibuster against any deal Democrats don’t support. “We are not doing anything into next year. Republicans should be aware of that right now.”

The potential stalemate over spending is a headache for Senate Majority Leader Mitch McConnell (R-Ky.) and House Speaker Paul D. Ryan (R-Wis.) who would like to avoid any whiff of a shutdown threat just weeks before the election. Neither has publicly weighed in on how long a stopgap spending bill should last, with aides saying it’s an issue that will be discussed with members when they return to Washington this week.

The opposition to a lame-duck spending deal in the House is being led by the approximately 40-member House Freedom Caucus with the backing of several prominent conservative groups. The picture in the Senate is less clear, but Sen. John Cornyn (R-Tex.), a member of leadership, has said he prefers kicking work on a final spending agreement into next year.

Conservatives have reason to worry that Democrats will try to use a yearend negotiation to force Republicans to accept additional spending or risk another shutdown fight weeks or days before Christmas.

Democrats have successfully used similar tactics several times in recent years, and Republicans have not found an effective strategy to fight back. But a yearend negotiation would also give GOP leaders one last shot at influencing federal spending in the event Republicans lose control of one or both chambers of Congress in 2017.

Republicans hold a slim 54-46 majority in the Senate, and many are worried they will face heavy losses in November thanks to the sinking popularity of GOP presidential candidate Donald Trump. House Republicans have a more comfortable advantage of 247-186, but they face the possibility of significant losses of their own.

Further complicating the budget debate is the continuing standoff over how to fund efforts to combat the spread of the Zika virus, which leads to birth defects.

Last month, Senate Democrats blocked a $1.1 billion spending bill over what they called poison pill provisions, including language that would deny Zika-related funds from being sent to Planned Parenthood and a provision that would loosen environmental regulations on pesticides. McConnell has said he plans to bring the same measure up again for a vote immediately when Congress returns. But Democrats still oppose the bill, and they are expected to block it again.

Congress will be under heavy pressure to address Zika funding as quickly as possible. Last month, Centers for Disease Control and Prevention Director Tom Frieden announced the agency would run out of funding for programs to combat the virus by the end of this month.

“The cupboard is bare,” he said. “Basically, we’re out of money, and we need Congress to act to allow us to respond effectively.”

That deadline makes it increasingly likely that Zika talks will merge with the larger spending debate.

“Despite Senate Democrats’ obstruction of the House-passed $1.1 billion bill to fight Zika, we are confident resources will get approved in September,” said Ryan spokeswoman AshLee Strong.

Republicans are expected to hold closed-door meetings throughout the week to gauge how willing members are to negotiate with Democrats on either issue. While lawmakers aren’t scheduled to recess again until the end of the month, Senate Republicans are under pressure to reach a solution sooner so vulnerable members can head back home to campaign. Twenty-two GOP senators are up for reelection in November.

The Senate could choose to negotiate its own spending agreement to speed things along. That would allow the Senate to vote on the must-pass legislation and leave town early, effectively forcing the House to either accept the Senate-passed bill or take the blame for a government shutdown.