WASHINGTON — The most partisan, least productive Congress in memory is bolting Washington for the campaign trail, leaving in its wake a pile of unfinished business on the budget and taxes, farm policy and legislation to save the Postal Service from insolvency.

The GOP-controlled House beat its retreat Friday morning after one last, futile slap at President Barack Obama — passing a bill entitled the “Stop the War on Coal Act.” The measure, dead on arrival with Obama and the Senate, would block the government from policing greenhouse gas emissions and give states regulatory control over the disposal of harmful coal byproducts.

Over in the Democratic Senate, Majority Leader Harry Reid, D-Nev., delayed that chamber’s getaway to force a procedural vote on legislation by endangered Democrat Jon Tester of Montana to boost access to public lands for hunting and fishing. Republicans protested that the move was nakedly political and had tried to block it.

The spitting match ensured a post-midnight Senate session before a final vote on the only must-do item on the agenda — a six-month spending measure to keep the government running after the current budget year ends on Sept. 30.

Reid also relented to a monthslong demand by tea party Sen. Rand Paul, R-Ky., for a vote on cutting off foreign aid to the governments of Libya, Egypt and Pakistan. The measure faced sure defeat. Sen. Lindsey Graham, R-S.C., however, was poised to win sweeping approval of a nonbinding resolution supporting action to make sure Iran doesn’t develop a nuclear weapon.

It’s the earliest pre-election exit by Congress from Washington since 1960, though lawmakers will return in November after the election to deal with its stack of unfinished work.

The approval rating for the current Congress in a Gallup poll earlier this month sank to just 13 percent, the lowest ever for an election year. The GOP-controlled House and Democratic Senate managed to come together with Obama to enact just 173 new laws. More are coming after the election, but the current tally is roughly half the output of a typical Congress.

Even so, political pundits say Republicans are strong favorites to keep the House while Democratic chances of keeping the Senate are on the upswing with Obama’s rise in the polls.

The exit from Washington leaves the bulk of Congress’ agenda for a postelection session in which it’s hoped lawmakers will be liberated from the election-year paralysis that has ground Capitol Hill to a near halt.

Topping the lame-duck agenda is dealing with the so-called fiscal cliff, which combines the expiration of the Bush-era tax cuts on Dec. 31 and more than $100 billion in indiscriminate, across-the-board spending cuts set to strike at the same time as punishment for the failure of last year’s deficit “supercommittee” to strike a deal.

Also left in limbo is the farm bill, stalled in the House due to opposition from conservative Republicans who think it doesn’t cut farm subsidies and food stamps enough and Democrats who think its food stamp cuts are too harsh.

The current farm act expires on Sept. 30 but the lapse won’t have much practical effect in the near term. Still, it’s a political black eye for Republicans, especially those from farm states like North Dakota and Iowa.

The lack of productivity of the 112th Congress was the result of divided government and bitter partisanship. The looming presidential and congressional elections caused top leaders in both parties to play it safe and stick to party positions.

The result: Congress’ major accomplishments tended to be legislation that mostly extended current policies, like a highway bill passed earlier this year and bills demanded by Obama to renew a 2 percentage point payroll tax cuts and extend student loan subsidies.

Even this Congress’ signature accomplishment — a budget and debt deal enacted last summer to cut $2.1 trillion from the budget over 10 years — punted most of its difficult decisions to the future by tasking the supercommittee with finding at least $1.2 trillion in deficit savings.

And, after the supercommittee cratered, House Republicans walked away from the budget deal by pressing for further cuts to domestic appropriations and reversing some on the pact’s Pentagon cuts.

Meanwhile, in the Senate, Reid worked closely with the White House to use the Senate schedule for Obama’s political advantage, repeatedly forcing votes on closing tax breaks for oil companies and raising taxes on upper bracket earners.

But Reid failed to schedule floor debates on any of the 12 annual appropriations bills and the Democratic-led chamber, for the third year in a row, failed to pass a budget.

Republicans also point to almost 40 items of House-passed jobs-related legislation sitting stalled in the Senate.

“They haven’t passed a budget in more than 3 years. They have no plan to save Medicare, no plan to stop all the tax hikes, and no plan to replace the sequester,” Boehner said. “This isn’t leadership. It is negligence.”

Democrats defending the Senate point out that the balky chamber managed several bills that the House would not, including a renewal of farm programs and legislation to overhaul the Postal Service and give it an infusion of cash to stave off insolvency.

“The reality is for as closely as divided as this Senate is, we passed a large number of bipartisan bills this year, very important bills, but as you all know, it takes two chambers to pass a law,” said Sen. Charles Schumer, D-N.Y. “On the other side, too many of the Congress members, particularly the tea party folks, think compromise is a dirty word.”