The Fix's Amber Phillips explains the tight deadlines Congress faces as they return from summer recess, and how President Trump's shutdown threat over funding his border wall and criticism of the debt ceiling "mess" threaten their agenda. (Jenny Starrs/The Washington Post)

Constructing a border wall is essential to President Trump — so essential, he said, he is willing to “close down” the government to get it.

But what would it take for Trump to actually force a government shutdown? And how likely is that to happen because of the border-wall debate?

As congressional staffers will tell you, it’s complicated. And nobody knows exactly how Trump would react in the moment of truth. Before now, he’s treated government shutdowns as not a big deal, even a good thing. It could be a lot harder to make that argument from the Oval Office.

Here’s a breakdown of the scenarios we could see playing out this fall — some leading to a shutdown, others keeping the government open past the Sept. 30 deadline.

Scenario 1: The government stays open because Congress passes funding for a border wall, giving Trump what he wants.

This is most straightforward — and least likely — outcome given the power of the filibuster and the strength of Democrats’ opposition to funding a wall.

In Trump’s ideal world, Republicans would include money to construct a wall in their next government-spending measure, and that measure would pass the House and Senate with overwhelming bipartisan support.

This won’t happen. Though Republicans hold majorities in both chambers, the GOP doesn’t have the 60 votes required to break a filibuster in the Senate. And Democrats would almost certainly exercise the filibuster to block a bill with wall funding.

Verdict: Less likely.

Scenario 2: The government stays open because Congress approves new border security measures that satisfy Trump, even though they don’t constitute full-fledged funding for a wall.

This case of bait-and-switch could serve to keep the government open, depending on how precise Trump wants to be about the definition of “wall.”

There are intermediate border-security measures that could gain support from both parties, potentially giving Senate Majority Leader Mitch McConnell (R-Ky.) the votes he needs to pass a spending bill that includes them.

President Trump threatened to shut down the government over building his promised wall on the U.S.-Mexico border during a campaign rally in Phoenix, on Aug. 22. (The Washington Post)

At that point, if Trump agreed, it would be up to him to finesse his rhetoric on the need for a physical barrier along the border. Could better equipment and technology be described as a “wall”? In Trump’s world, it could happen. But it’s another matter whether his base voters would accept it.

Verdict: More likely.

Scenario 3: The government stays open because Congress passes a short-term spending bill, promising to deal with wall funding at a later date.

“Kicking the can down the road” tends to be Congress’s preferred way of dealing with difficult problems, and this scenario could be no different.

As the clock ticks down toward Sept. 30, lawmakers will face increased pressure to fund the government, even if just on a temporary basis. By promising to debate wall funding later, then approving a stopgap spending measure, Congress would put off a confrontation with Trump and the possibility of a shutdown.

The downside of procrastinating is it would complicate lawmakers’ already jam-packed legislative schedule through the end of 2017. The upside? Less trouble in the short term. This approach historically has held major appeal for lawmakers, increasing its likelihood.

Verdict: More likely.

Scenario 4: The government stays open because McConnell breaks the legislative filibuster, allowing funding for a wall to pass.

Trump would love for McConnell to get rid of the legislative filibuster the way he did the filibuster on Supreme Court nominations. But it would require convincing more than two dozen Republicans to reject an opinion they held just a few months ago.

In April, 61 Republican and Democratic senators wrote to Senate leaders urging them to preserve the 60-vote threshold for legislation.

“Senators have expressed a variety of opinions about the appropriateness of limiting debate when we are considering judicial and executive branch nominations,” the letter stated. “Regardless of our past disagreements on that issue, we are united in our determination to preserve the ability of Members to engage in extended debate when bills are on the Senate floor.”

Verdict: Less likely.

Scenario 5: The government shuts down because Congress approves a spending bill without wall funding and Trump makes good on his threat.

It’s possible the president will hold Congress’s feet to the fire and reject any spending bill that doesn’t include funding for the wall. In that case, Trump would have to be ready to risk the political consequences of a shutdown — consequences we know he was aware of because of something he tweeted when the government shuttered last time.

“My sense is that people are far angrier at the President than they are at Congress re the shutdown — an interesting turn!” Trump wrote in October 2013.

Verdict: More likely.

Scenario 6: The government stays open because Congress manages to overcome Trump’s veto of a spending bill without wall funds.

This would be difficult for Republican leaders to achieve, but not impossible. Say Republicans pass a spending bill without wall funding and Trump vetoes it. If McConnell and House Speaker Paul D. Ryan (R-Wis.) can convince two-thirds of each of their chambers that keeping the government open is more important than funding the border wall, they could overcome the veto and keep the government open.

Verdict: Less likely.