I don’t see anything criminal in a party official telling a candidate that the other candidate has a better chance of winning, and that he should drop out and avoid a primary battle.
What’s criminal about it?
Nothing, if you think the party-identifying voters shouldn’t have any role in selecting the candidate for the seat.
If that is the case, why hold a primary election, at all? “Winning” the primary election would then be nothing more than a bit of small “d” democratic-process fiction papering over the Democratic Party’s dictatorship.
American politicians are good at that: honoring in solemn pronouncements the virtues of a supposedly neutral process while simultaneously subverting it. The 2016 Democratic Party Presidental primary season being a stellar example.
What’s criminal about it?
Probably it would only be illegal if there was an expressed payoff for dropping out (support us in this race and I'll see to it that you'll get a plum position in the DCCC for the next election cycle, etc...), and experienced politicians are rarely dumb enough to do that, at least face to face. An associate or donor may call later and 'have a great opportunity' later, but it's all plausibly deniable by the actual politician.
The real crime caught on tape here is moral, not legal. It is rampant on both sides as Hoyer admits, and it is poison to the Republic.
The GOP has clearly been interfering in primaries for years to keep their establishment incumbents in place and avoid serious primary fights. What they never seem to understand is there is such a distaste for establishment politicians, that rank amateurs and absolute wildcards take the place of better options. When they win, the whole party is smeared as a bunch of loons and the establishment doubles down on the same losing bet.
I hope this does end up going viral and leading Republicans get shamed into backing out of their own primary interference activities.