If the Republican majorities in the House and Senate are unable to get legislation to President Donald Trump’s desk to keep the government running beyond an April 28 deadline, it could be a fairly historic political moment.

Not since President Jimmy Carter’s administration have a Congress and an executive branch unified under one party seen government funding gaps occur, according to the Congressional Research Service.

That was despite one funding gap during Carter’s administration that lasted 17 days — one of the longest appropriations lapses in the history of the modern budget process.

But hashing out a deal on fiscal 2017 — the most immediate political negotiation that could lead to a shutdown if unsuccessful — isn’t so simple for the GOP majorities who control today’s Congress either.

A highly conservative faction in the House that wants to see its priorities enacted in spending bills grates against a Democratic Senate minority that has the ability to block advancement of most legislation. The Senate needs 60 votes to advance spending bills and Republicans only hold 52 seats in the chamber.

The White House’s ask for $18 billion in additional fiscal 2017 cuts is objectionable to Democrats, as is increased spending on defense and a controversial U.S.-Mexico border wall.

That is a source of headaches for Republican appropriators, too, who see the Trump requests as fairly late, given that fiscal 2017 efforts began with President Barack Obama’s budget request, and that the fiscal year will be more than halfway over by the time any legislation is expected to be signed.

Awaiting members as they return this week is a mountain of spending work that includes 11 unfinished fiscal 2017 bills, the return of sequester-level spending caps in fiscal 2018, and the need to raise the debt limit sometime later this year.

A rare event

There are several explanations for why a government shutdown under unified Republican government would be so unusual. For one, voters have rarely kept one party in control of both the legislative and executive branch for very long in recent history.

The last instance of unified Republican control was during President George W. Bush’s administration. That preceded the tea party wave that laid bare deep divisions between conservative and moderate Republicans.

Today, the moderate-conservative dynamic between Republicans poses a greater threat to consensus on legislation within the House and complicates bicameral agreement with the more centrist Senate.

In addition, recent shutdowns — which led to furloughed workers and shuttered national parks — were guided by legal opinions from Carter’s attorney general, Benjamin R. Civiletti, issued in 1980 and 1981. He more strictly interpreted a federal law that prevents outlays in the absence of appropriations.

Described in a 1981 report to Congress, researchers mark this administration action as a turning point for how the cessation of government operations was handled.

Previously, the CRS found, lapses in appropriations — what they termed “funding gaps” — didn’t trigger massive shutdowns, especially not like the shutdowns that hamstrung the federal government from December 1995 to January 1996 and in the fall of 2013.

In both cases, demands from Republican majorities clashing with a Democratic president led to a shutdown: 21 days under President Bill Clinton, and 16 days under Obama. While both the House and Senate were controlled by the GOP during the Clinton shutdown, the Obama shutdown occurred with a Republican House and a Democratic Senate.

Blame game

As the end of a continuing resolution looms, once again shutdown blame has become a repeated topic of conversation.

“Our Republican colleagues know that since they control all … the House, the Senate and the White House, that a shutdown would fall on their shoulders and they don’t want it. We want to make sure it’s a good budget that meets our principles, but so far, so good,” Senate Minority Leader Charles E. Schumer said during a press call Tuesday, when asked about the shrinking window of time for lawmakers and the president to reach agreement on spending.

But Senate Majority Leader Mitch McConnell has repeatedly suggested that Democrats and Republicans in Congress — not the president — would be blamed for a shutdown.

“I’m amused by the Democrats apparently warming up to the idea that threatening to shut down the government’s a good idea. It seems to me everybody’s got kind of memory loss on [the] other side. They will be players when it comes to the appropriations process,” the Kentucky Republican said when asked in an interview in March.

Pressure is high for the GOP to deliver a spending bill before current funding runs out, as Republicans in both chambers struggle to recover from a setback on using budget reconciliation to partly repeal and replace the 2010 health care law.

There will also be tough optics to square if there’s any delay on funding past the deadline as the expiration of the continuing resolution falls on Trump’s 100th day in office.

But thorny spending issues are at play with less than a week to iron problems out when lawmakers return from their two-week recess. The Senate is in on Monday and the House returns Tuesday.