WASHINGTON — Congress is heading out for a five-week summer recess in anything but a cheerful vacation mood, leaving behind a pile of unfinished business that all but guarantees a painful fall.

Not long after they return in September, lawmakers must vote on President Obama’s nuclear deal with Iran, a brutally divisive issue that many lawmakers expect will dominate voter town halls during their annual August break.

And, as more videos emerge showing disturbing fetal tissue collection practices, Republicans are increasingly focused on cutting off funding for Planned Parenthood, raising the prospect that Congress will spend September tied in knots over how to avoid shutting down the government over that issue.

Later in the fall or winter, Congress will have to raise the federal debt limit, another issue ripe for brinkmanship, especially given the presence in the Senate of several presidential candidates adamantly opposed to an increase.

The House wrapped up its summer session by approving only a three-month extension of highway and transit spending and authority, kicking negotiations on that into the fall, as well.

Add in deadlines to renew authorities for the Federal Aviation Administration, child nutrition standards and pipeline safety, and it’s shaping up as a monster of a fall.

“If you take a look at all of the things on the list, it’ll be a lot of traffic going through one toll booth,” Sen. John Barrasso, R-Wyo., said Thursday.

Republican Rep. Mick Mulvaney of South Carolina, who leads a group of 18 conservatives vowing to oppose any spending bill that funds Planned Parenthood, says, “This is one of those line-in-the-sand-type of issues. … We have to figure out a way to fund the government without giving any more money to this institution.”

The effort could prevent leaders from extending current spending levels come the new budget year Oct. 1, since Planned Parenthood now receives more than $500 million in government assistance. Yet if Republicans try to use must-pass spending legislation to kill off the organization’s funding, they would have trouble getting past Senate Democrats and the White House. And that could leave Republicans who took control of Congress this year promising to avoid shutdowns and “fiscal cliffs” backed into a very uncomfortable corner.

“Democrats will unite against them,” says Chuck Schumer of New York, the No. 3 Senate Democrat. “This is a Republican path to shutdown.”

While the House left town Wednesday, the Senate plans one more week before leaving, with a cybersecurity bill and a largely symbolic vote on defunding Planned Parenthood.

Along with Iran, the government spending issue tops a long list of thorny disputes that threaten to have Republicans and Democrats at loggerheads for months.

The 12 annual spending bills that fund the government are hung up on a variety of disagreements, including a dust-up in the House over the Confederate flag and Democrats’ demands to increase domestic spending. That leaves Congress facing the likelihood of temporarily extending current spending levels, which gets lawmakers back to the prospect of a shutdown showdown over Planned Parenthood.

On Iran, Republicans are largely united against the nuclear deal, while those Democrats who’ve not yet declared their position are under enormous pressure from both sides. The White House is imploring them to back the president, while groups allied with the Israeli government are warning against the deal in apocalyptic terms. Congress is widely expected to vote down the deal, at which point attention would turn to whether opponents could muster the two-thirds vote in each chamber to override Obama’s certain veto.