Congress returns this week staring down a pressing deadline: Both the House and Senate need to approve — and then agree on — a series of spending bills by the end of September or the government could go into a shutdown. Again.

The tight timing means that passing these bills will be a chief priority for lawmakers as they come back from the August recess, with just three weeks to go before existing government appropriations expire on September 30. As things currently stand, the House has already approved 10 out of 12 spending bills, while the Senate has not moved on any at all.

Lawmakers in the Republican-controlled upper chamber previously said they were holding off on further spending action until Congress and the president were able to agree on a budget plan. Now that the budget has been squared away, the Senate is expected to begin addressing its appropriations backlog, though it’s unlikely it will be able to do so in full before the looming deadline.

Instead, Congress is expected to pass a short-term spending measure, known as a continuing resolution, in order to keep the government funded and prevent another shutdown from taking place. That measure would likely maintain most spending levels at their prior-year allocations and buy both chambers some time to reconcile their differences on these bills. Potential funding for a wall along the southern border is among the line items where the two chambers could differ.

Following the historic shutdown that took place earlier this year, there’s little appetite from lawmakers on either side of the aisle to have another one. Preventing that from happening is set to be a chief goal in both the House and the Senate this month.

What Congress still needs to do on appropriations

Every year, Congress is required to approve spending bills for the upcoming fiscal year before October 1. That includes 12 bills dedicated to funding different parts of the government, broken out below:

Agriculture

Commerce, Justice, and Science

Defense

Energy and Water

Financial Services and General Government

Homeland Security

Interior and Environment

Labor, Health and Human Services, and Education

Legislative Branch

Military Construction and Veterans Affairs

State and Foreign Operations

Transportation and Housing and Urban Development

The House has already passed nearly all of these bills, barring the legislation funding the Department of Homeland Security and the Legislative Branch, which includes money for the House of Representatives, the Senate, and the Congressional Budget Office. The Senate, meanwhile, is set to get started on approving its versions beginning with a markup on September 12.

Once both chambers have approved their version of these bills, they’ll have to reconcile their approaches, a process that could take significant time to finish given differences in priorities between the Democratic-controlled House and the Republican-controlled Senate. The White House has already said Trump would probably veto spending packages previously approved by the House this year, including one that includes more funding for research on gun violence.

The chances of both chambers sorting through their differences in three weeks is slim. But in order to avoid a potential shutdown this fall, House majority leader Steny Hoyer has said they’ll likely consider a short-term continuing resolution.

Hoyer previously floated this option, which would keep the government fully funded through November 22, during a call with other House Democrats last week, according to Politico.

“Mr. Hoyer’s preference is to fund the government in September, but the Senate, for the first time in decades, hasn’t marked up a single appropriations bill in subcommittee,” Mariel Saez, a Hoyer spokeswoman told Government Executive. “While the House has acted and wants to go to conference with the Senate on appropriations bills as soon as possible, there is a possibility that we will need a short-term CR to provide time for the Senate to do its work.”

Border wall funding could be a sticking point for long-term appropriations bills

This past winter, a fight over funding for President Donald Trump’s border wall was the central cause of the longest government shutdown the country has ever experienced. And this time around, it could continue to pose an issue.

While Trump ultimately caved in February and reopened the government after Democrats refused to give him any money for the border wall, he has since gotten around that roadblock by declaring a national emergency and shifting funds from other military projects. As Vox’s Alex Ward reports, the administration is expected to move funds affecting 127 military construction projects including improvements to the US Military Academy in West Point and a US National Guard readiness center in Puerto Rico.

The administration’s push for roughly $8.6 billion in wall funding is set to be a sore subject when it comes to negotiations around the 2020 spending bills. As Roll Call previously reported, Senate Appropriations Chair Richard Shelby is considering reducing spending in Labor and Education appropriations in order to cover such possible outlays for the wall.

These maneuvers might, once more, provoke opposition from Democrats. Whether they wind up stalling spending approval yet again remains to be seen.