The US government could go into partial shutdown this weekend, raising the specter of mass employee furloughs and the suspension of many federal services if the situation goes unresolved.

A best-case scenario would see lawmakers secure a last-minute deal before the shutdown deadline of midnight Friday –- or organize a vote early next week to keep any closure short and with minimal impact.

Here is what could happen if Congress fails to do so:

Who would be impacted?

Congress has already passed six of twelve funding bills, with the outstanding six covering agencies including the Departments of Homeland Security, Health and Human Services, Labor and Defense.

Tens of thousands of federal workers in the affected departments would likely be furloughed, or work without pay during a government shutdown.

The Department of Defense would be the largest one to be affected -- despite most of its workers being deemed essential, meaning they cannot be sent home in the event of a shutdown.

Of the 800,000 or so civilian employees at the agency, around 440,000 -- more than half -- would be sent home without pay, according to Andrew Lautz, a senior policy analyst at the Bipartisan Policy Center in Washington. 

If the shutdown is short-lived, "there are disruptions, but not huge disruptions," he told AFP.

At the Treasury's Internal Revenue Service (IRS), more than half of the almost 90,000 employees could also be forced to stay home under contingency plans announced last week, hampering the bureau's efforts to process Americans' tax returns just weeks before the April 15 filing deadline. 

"You can't avoid having knock-on effects to the entire workforce when you take away that large of a chunk overnight," Lautz said. "But that being said, the IRS clearly has a plan here."

Unlike a previous shutdown in 2019, this one would not lead to the closure of Smithsonian museums and America's much-loved national parks, as they were funded as part of another funding agreement on March 8.

How long could a shutdown last?

If the US Senate can pass the funding bill on Friday, a shutdown is averted, and the government would continue to be funded through September 30 this year.

But if the bill fails to pass by the deadline, the shutdown is unlikely to be a long one, economists at Citi wrote in a recent investor note.

"Congress is very likely to either avoid a shutdown entirely or limit it to a technical shutdown during the weekend that would not significantly affect government functions," they said. 

After some last-minute brinkmanship, the Republican-led House of Representatives passed the $1.2 trillion bill just before noon local time Friday (1600 GMT), sending it to the Senate for a vote. 

Party leaders fear the Senate may struggle to rubber-stamp the House vote before the midnight deadline, with arcane procedural rules threatening to push the action into the weekend.

However, the shutdown deadline comes just before Congress is scheduled to shut up shop for two weeks over Easter, and senators have shown little appetite for a weekend of drama or a delayed start to the holiday.

As a result, they may still agree to hold a last-minute vote on Friday and avert a shutdown -- buying themselves a few quiet months before negotiations restart over funding levels for the next fiscal year, which starts on October 1.

"Over the last 40-plus years, Congress has been on time with its budget four times," Lautz said, adding that lawmakers are unlikely to agree on a funding deal for next year in the run-up to the 2024 elections in November.

"I think it's very likely -- not certain but very likely -- that they punt, at least past the election," he added.