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 Barack 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.