Donald Trump has had a pretty amazing run over the last few months, as he's soared from an asterisk in polling to the top of the GOP presidential field. But, increasingly, Trump seems to be a candidate whose shtick of bravado and bluster is running out of gas, even as it becomes increasingly clear that he doesn't have a second act (or hasn't come up with one yet.)

Take the last 24 hours. Trump called in to "Meet the Press" and was asked by moderator Chuck Todd whether he believed a Muslim could or should be president. Here's how Trump answered:

I can say it’s something that at some point could happen. We’ll see. I mean you know at some point it could happen. Would I be comfortable? I don’t know if we have to address it right now but I think it’s certainly something that could happen. ... Some people have said it already happened, frankly, but, of course, you wouldn’t agree with that.

The insinuation that President Obama is some sort of secret Muslim is a conspiracy theory that has been kicking around the Internet for years. But, for a leading presidential candidate to just throw it out there on national TV?

Then came this Instagram video released by Trump on Monday in which he goes after Jeb Bush.

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Jeb has been confused for forty years...

A post shared by President Donald J. Trump (@realdonaldtrump) on

The wing, wink, nudge, nudge implication of the video is, of course, that some of Bush's less-popular policy proposals -- like his support for comprehensive immigration reform -- come because he is smoking pot, an act he owned up to in high school.

Personal insults and the spreading of barely credible (or not-at-all credible) information are not new for Trump. Much of his campaign has subsisted on his willingness to denigrate any and every opponent. Far from alienating voters, it has attracted them.

But there is some evidence that Trump's act is wearing thin even among voters who were initially attracted to his bombast. Look at this chart documenting the ups and down of the Republican field dating back to late July.

Trump's dip began before the Sept. 16 national debate that CNN sponsored. But it's also true that the debate did little to slow his fall. (The poll was released Sunday and was in the field after the debate concluded.)

Think of Trump's appeal like a new song. You hear it on the radio -- or, more likely on iTunes or Spotify -- and listen to it a bajillion times. A week or two weeks later, you start to get a little sick of it and buy a few more songs by the band who made it. Except none of those songs are nearly as good as the one you loved so much. So, you move on -- looking for new music.

Carly Fiorina, Marco Rubio or even, to an extent, Ben Carson might well be the new music that people find as they look beyond Trump. Or, it is of course possible that Trump finds a second act -- or has one up his sleeve that he isn't showing at the moment. (Important note: I didn't even think Trump would have one act, so he's way ahead of me already.)

But if Trump's performance in the second debate -- and what he has said/how he has acted since -- is any guide, I count myself as skeptical that he will be able to launch a more substantive phase of the race. He has one speed -- full ahead ahead, at all times. Eventually that starts to wear on the American public's engine. That's where Trump is today.