(CNN) Without the border wall, Donald Trump might have only been a gold-matted asterisk in the 2016 presidential race. But with the border wall, the longshot real estate billionaire went from political purgatory to 1600 Pennsylvania Avenue.

Given the centrality of his pledge to build a wall along the southern border of the United States (muffled voice: And make Mexico pay for it), Trump is totally committed to ensuring that he can go to voters in 2020 and tell them he made good on that most important of promises. Which is a big, big, big problem for Republicans desperately trying to hold onto their House and Senate majorities in 99 days' time.

"I would have no problem doing a shutdown," Trump said bluntly during a joint press conference Monday afternoon with Italy's prime minister. That bombshell confirmed a Trump tweet from over the weekend. "I would be willing to 'shut down' government if the Democrats do not give us the votes for Border Security, which includes the Wall," Trump tweeted on Sunday . "Must get rid of Lottery, Catch & Release etc. and finally go to system of Immigration based on MERIT! We need great people coming into our Country!"

Those twin pronouncements will land like a rotting fish head at a dinner party for the likes of Senate Majority Leader Mitch McConnell and House Speaker Paul Ryan.

For every second since the last time the government shut down -- in January -- McConnell has insisted it would not happen again.

"The Senate has a lot to do this week," he said from the Senate floor just moments after Trump made his shutdown threat Monday. "We'll finish up the set of appropriations measures we've been considering for several days and take four more big steps towards our goal of completing a regular appropriations process and funding the government in a timely and orderly manner."

McConnell's quote is the essence of envisioning the best possible result in the future, repeating it as much as possible and hoping that it comes true.

McConnell figures if he says "no shutdown" enough, he will keep it from happening. But wishful thinking is not legislation that funds the government. And, with only 11 -- yes 11! -- legislative days between now and the September 30 government shutdown, McConnell knows better than anyone how politically dangerous Trump's threats actually are.

Let's count the reasons.

1. Republicans control all levers of government

Government shutdowns have one proven effect -- they make people pay attention to Washington. And it's almost never a good thing to remind the average American of how dysfunctional the nation's capital is and how much they dislike it. There tends to be a bit of a "pox on both your houses" reaction to a shutdown, but that typically dissolves into blaming the side perceived to have brought the shutdown about. And no matter what Trump says or does between now and September 30, there's one thing that he can't change: Republicans have the White House as well as majorities in the House and Senate. Simple arguments tend to work best, and Democrats have the simpler case to make: Republicans control everything, so it's their job to get something done.

2. Distracting from the economy

The best thing that Republicans trying to hold the majority in the House and Senate have going for them right now is the current economic numbers (4.1% economic growth in the second quarter of 2018) and the growing belief among voters that things are well on their way to getting better. One example: Even though Trump's overall approval rating is in the low 40s in a new NBC-Wall Street Journal poll, his approval for handling the economy is over 50%. The path for Republicans to minimize their losses in November is to talk relentlessly bout how well the economy is doing. Anything that distracts from that message is trouble. And aside from perhaps the findings of special counsel Robert Mueller, there's almost nothing worse for Republicans to be talking about/defending than why they shut the government down.

3. The timing

Shutting the government down on January 20, 2018 is one thing. Shutting it down on October 1, 2018 is another. The first day of this government shutdown would occur just 36 days before the midterm elections. That sort of close proximity would ensure that the shutdown would be on the minds of voters on November 6. And, per No. 1 above, that is almost certainly a loser for congressional Republicans.

4. Trump's unreliability as a negotiator

In Monday's press conference, a reporter with The Daily Caller, a conservative media website, tried to nail Trump down on what, specifically, he would be asking for to keep the government open. Was it $25 billion in federal funding for the border wall? Trump refused to answer. "We need a border," he said. "Border security includes the wall but it includes many other things. I would certainly be willing to close it down to get it done." The problem for congressional Republicans in that statement is that there is no road map to avoiding a shutdown. There is no sign of whether Trump will take a half (or quarter) loaf on border wall funding or whether the government shuts down if he doesn't get the whole $25 billion. And that's not even getting into what Trump would be willing to accept on family-based immigration or some sort of lasting solution for Deferred Action for Childhood Arrivals recipients.

Make no mistake: McConnell, Ryan and every other Republican in Congress who likes being in the majority will try to talk Trump down off his latest shutdown threat. The problem is what, exactly, can they offer him that will placate him? And even if it it makes him happy on September 27, will it keep him happy on September 30?