The president of the United States just called into question the legitimacy of November's midterm elections.

That's a remarkable thing from any U.S. president, but it's not as surprising coming from President Trump.

He repeatedly called his own presidential election rigged, at least before he won. The likely motive then wasn't hard to decipher: The polls suggested Trump was going to lose to Hillary Clinton, and preemptively calling the election “rigged” was one way to rationalize his expected loss.

This time, it's less clear what Trump means when he warns that there will be “meddling” in the November midterm elections.

There is no evidence to support what the president is saying. But his tweet Tuesday morning and his past comments give us some clues about how he came up with this new conspiracy theory regarding the special counsel's Russia investigation.

1. Trump thinks the Russia probe isn't going to end anytime soon: The 13 Angry Democrats … will be MEDDLING in the mid-term elections.”

"13 Angry Democrats” is just one of the many ways Trump regularly tries to discredit special counsel Robert S. Mueller III's probe into whether Trump's campaign helped Russia interfere in the 2016 election.

As The Washington Post's Matt Zapotosky explains, Trump is probably referring to 13 lawyers on Mueller's team who were previously registered as Democrats. The president is making lots of assumptions when he declares that these lawyers, who were vetted by people appointed by Republicans, are conspiring against him because of their political views.

Four other attorneys on Mueller's team of 17 had no affiliation or their affiliation couldn't be ascertained, and Mueller himself is a Republican.

Those nuances don't seem to matter much to Trump. The discrediting of the probe and the new election conspiracy theory suggest Trump thinks the Mueller investigation may not be finished by the time voters get a say on his party and, by proxy, his presidency.

That could be a problem for Trump. No president wants to be under investigation when his party is on the ballot — least of all for potentially colluding with Russia.

While recent polls have found that a shrinking majority supports the Russia investigation, a majority of Americans still do, including all-important independent voters.

2. Trump may not sit down for an interview with Mueller: No one but Mueller and his team — and maybe not even them — knows when the Russia probe will end.

We have clues, though. It is general practice in the Justice Department not to escalate investigations that could interfere with U.S. politics, and that has led some (including on Trump's team) to believe the investigation will wrap up before November.

But Mueller may see a reason to keep going into November. For one, he's far enough along in his investigation that he wants to interview the president — so much so that Mueller has threatened to take Trump to court to force him to sit down for an interview.

If Trump doesn't agree to sit down, that could lead to a lengthy court battle, which could extend the length of the probe.

Since Robert S. Mueller III was appointed in May 2017 to investigate the Trump campaign's ties to Russia, President Trump has relentlessly attacked him. (Jenny Starrs/The Washington Post)

Although Trump probably has no insider knowledge into when the Russia investigation will end, he does have a sense of whether he will sit down with Mueller.

Trump's tweet Tuesday could be interpreted as a signal that he is not going to sit down with Mueller and that therefore, the Russia probe isn't going to wrap up anytime soon. With the knowledge that the Mueller probe could get more dramatic rather than less as November gets closer, the president could be trying to get out ahead of that by equating the probe with “meddling” in the election.

3. Trump might think he's going to lose: The last time Trump alleged that a U.S. election was rigged or illegitimate was when it looked as if he was going to lose.

Trump isn't on the ballot in November, but many Republicans in Congress, governors' seats and state legislatures are.

Conventional political wisdom says the party that is in power loses seats in midterm elections. This time, that could be exacerbated by Trump's extreme unpopularity on the left, which could motivate Democratic voters to do something they rarely do in midterm elections: Turn out in as high or greater numbers as Republican voters.

It's possible Trump is reading those tea leaves and looking around for a reason to cast doubt on the results of an election in which his side potentially loses.