With the phrase floating around on Monday, the Washington Post’s Philip Rucker asked President Trump to define it at the White House coronavirus press conference:
Rucker: In one of your Mother’s Day tweets, you appeared to accuse President Obama of “the biggest political crime in American history, by far” — those were your words. What crime exactly are you accusing President Obama of committing, and do you believe the Justice Department should prosecute him?
Trump: Obamagate. It’s been going on for a long time. It’s been going on from before I even got elected, and it’s a disgrace that it happened, and if you look at what’s gone on, and if you look at now, all this information that’s being released — and from what I understand, that’s only the beginning — some terrible things happened, and it should never be allowed to happen in our country again. And you’ll be seeing what’s going on over the next, over the coming weeks but I, and I wish you’d write honestly about it but unfortunately you choose not to do so.
Rucker: What is the crime, exactly, that you’re accusing him of?
Trump: You know what the crime is. The crime is very obvious to everybody. All you have to do is read the newspapers, except yours.
This is what I meant when I said that Trump couldn't articulate what crimes Obama has committed. The idea that the Obama administration illegally investigated Trump while campaigning for the Democratic Party candidate didn't come from Trump himself but rather conservative media.
Trump knows that he doesn't have to specific nor accurate in his claims in order for his supporters to agree with them, so he has no reason to be. These people will side with Trump no matter how outlandish, vague, or inaccurate his claims are.