It was one of the first promises Donald Trump made when he declared his candidacy for president, one he kept repeating as an applause line to raucous crowds throughout his campaign: He was going to build a wall along the Mexico border. And not only that: He promised Mexico would pay for it.

"I would build a great wall, and nobody builds walls better than me, believe me, and I'll build them very inexpensively," he said in 2015. "I will build a great great wall on our southern border and I'll have Mexico pay for that wall."

It was a bad promise back then. It's a broken promise now.

Two years into Trump's presidency, the wall still doesn't exist — though he keeps lying about that simple fact — and Mexico is understandably no closer to picking up the tab. So now, President Trump is looking to the American taxpayers to do the job that Mexico won't, asking Congress to pony up $5 billion so he can finally start building his precious border barrier.

If he doesn't get what he wants, Trump says he'll shut down the government when funding expires Dec. 21.

Let him.

Trump thinks he holds all the cards with voters on the immigration issue, and it's true that his base responds positively to calls for increased border security. But his attempt to pressure House Minority Leader Nancy Pelosi (D-Calif.) and Senate Minority Leader Chuck Schumer (D-N.Y.) into helping him secure the $5 billion is failing — liberal Democrats might go into open rebellion if they did so. Trump simply doesn't have the leverage he thinks he does.

Democratic leaders have two factors on their side: The first is that Trump is probably bluffing: We've seen him back away from similar threats before, and there's no reason to take him seriously this time. The second factor is that Dems can simply present the shutdown as a result of the president's inability to keep his most famous promise.

"The president promised you wouldn't pay for the wall," they can tell the country. "Now he's decided to bring government to a halt because he couldn't keep that promise. Is that fair?"

In truth, the border wall was always a bad idea — immoral, disruptive to landowners, a likely boon for drug cartels and, after all of that, still probably ineffective. Those reasons are good enough for Democrats to refuse to help the president on wall funding.

The suggestion that Mexico would pay for the wall was even worse: Nobody outside of Trump's circle has ever believed that country would keep Trump's promise for him.

But the idea was also extremely odd, if you think about it. When you build a fence on your own land, you don't make your neighbor pay for it — you probably wouldn't even think to ask. So why the appeal? As with most things in Trumpworld, a bully's logic makes the most sense here, the wrongheaded idea that Mexicans would pay for our wall because we are bigger and stronger and can make them bend to our will. For Trump, this was a show of strength and swagger, and it turned out to be hollow.

Trump should be on the ropes with this issue. Unfortunately, Democrats have given him a ray of hope.

Schumer and Pelosi, it turns out, have offered their party's congressional support to budget up to $1.3 billion for the border wall. That's roughly $1.3 billion more than most Democratic voters would prefer. Still, Trump says it's not enough — he wants the full $5 billion, or the government shuts down. But on Monday night, Pelosi signaled she's willing to go no further:

"Our country cannot afford a #TrumpShutdown, especially at this time of economic uncertainty," she tweeted. "This holiday season, @realDonaldTrump knows full well that his wall proposal does not have the votes to pass the House & Senate, and should not be an obstacle to a bipartisan agreement."

That's the right, hardline stance to take. The border wall sums up how Trump has approached the presidency in general: He had a bad idea, failed to execute it properly, and now threatens to throw a tantrum if he doesn't get his way. Democrats shouldn't enable Trump's broken promises — in this case, at least, they should stand back and let him fail.