Taking Trump at his word is always a risky proposition. But there are several reasons this threat is different from all those that proceeded it.
First, we are just past the midterms and far away from the 2020 presidential election. With no chance of immediate electoral impact, any panic from Senate Majority Leader Mitch McConnell (who once called himself “the guy who gets us out of shutdowns”) can be more easily ignored by Trump.
Second, with Trump increasingly on the defensive thanks to the investigations of foreign influence on his campaign, inauguration and administration, he is eager to go on the offensive somewhere and rally his political base. What could be more satisfying to that base than a battle royal for his signature campaign promise?
Third, because Trump and Congress have already approved spending for some government agencies, what awaits us is not a complete government shutdown but a partial one. And some affected agencies would still be able to limp along.