It's hard to imagine Hillary Clinton having a much better day than she did on Tuesday. Voters in Florida, Ohio, Illinois, North Carolina, and Missouri went to the polls — and on the Democratic side, they delivered at least four of those states to Clinton. (Clinton leads Bernie Sanders in Missouri by 0.2 percent, or 1,531 votes; a recount is possible.)

This is a big blow to the Sanders revolution, both in the delegate race and for its narrative that Clinton is a regional candidate who has trouble winning outside of the South. After Sanders' big upset win in Michigan a week ago, his campaign was cautiously optimistic it would win demographically and economically similar Ohio and Illinois, plus Missouri. Clinton was on the ropes. But despite tightening polls and an aura of momentum, Sanders was soundly defeated in Ohio and narrowly lost in Illinois. Politico's Glenn Thrush not too kindly pointed to the Simon and Garfunkel song adopted by the Sanders campaign:

So what went wrong for Sanders? Well, you could argue that nothing really went wrong — the polls had him losing to Clinton, and he did. Judging Sanders' March 15 losses by his March 8 triumph in Michigan doesn't really seem fair. Illinois is Clinton's birth state, and she only won it by 1 percentage point.

But Sanders needed to win big if he wants a real shot at the nomination, and instead he lost — even after outspending Clinton on ads in Ohio. The Vermont senator is doing a great job of winning on issues that are fueling his campaign, but he's getting close to being only an issues candidate, not a viable nominee.

Here's one plausible explanation for Sanders' electoral setbacks: It's all about Donald Trump, who had almost as good a night as Clinton, winning in Florida, Illinois, and North Carolina.

Trump has dominated the news recently for allegations that he incited violence at his rallies. He publicly toyed with paying the legal bills of a supporter who sucker-punched a protester, for example, and campaign manager Corey Lewandowski is facing high-profile accusations that he manhandled a reporter from a friendly conservative news site.

Bad press is nothing new for Trump, but this week's batch had a sinister edge to it. That didn't hurt him among Republican voters — it may have even helped him — but Democrats and some Republicans are disturbed that Trump's violent rhetoric and authoritarian posture hasn't slowed his gallop to the GOP nomination. If you are a Democratic primary voter and you are on the fence between Clinton and Sanders, fear of Trump may well be enough to tip the balance toward Clinton, who seems like a surer bet to take down Trump in the general.

There is some evidence to back up this theory. The first is the exit polls from Tuesday's election. There are arguments that Sanders is in a better position to beat Trump in a general election, but Democratic voters don't seem to be buying them. Clinton has always led Sanders among voters who say electability is a key attribute in the Democratic nominee, and two-thirds of Democratic voters on Tuesday said Clinton is more electable than Sanders.

The states where she did best, like Florida, had a higher percentage of respondents (75 percent) calling Clinton more electable than in the states that were much closer, like Missouri (57 percent). The Democratic electorate found Sanders more inspiring and honest, but a fat lot of good that will do if Trump is elected president. Even if electability isn't a voter's primary concern for a presidential candidate, it almost certainly factors in, especially if he or she views the probable alternative as completely unacceptable or even dangerous.

Here's another bit of evidence that Trump may be helping Clinton: President Obama's approval rating has risen to a three-year high of 51 percent, according to Gallup's tracking poll. Gallup says most of Obama's improved fortunes are due to rising support among Democrats, and posits that "the unusual status of the Republican primary race — exemplified in particular by frontrunner Donald Trump's campaign style and rhetoric — may serve to make Obama look statesmanlike in comparison."

Or as Noah Berlatsky puts it, "Democrats look at Trump and feel better about their party leaders in comparison." And Clinton is the candidate who has most aligned herself with Obama in the 2016 race. According to exit polls, 53 percent of Tuesday's Democratic primary voters wanted their nominee to continue Obama's policies, and about 70 percent of them voted for Clinton (Sanders did better among the 31 percent who wanted to go in a more liberal direction).

Elections don't happen in a vacuum, and whatever the reason, Clinton won (or tied) Sanders in five very different states on Tuesday, just one week after Sanders shocked the Clinton campaign and political press in Michigan. What changed? Donald Trump is as likely an explanation as any.