The jury conviction of Paul Manafort and the guilty plea by Michael Cohen on Tuesday are a damaging commentary on the shady operators Donald Trump associated with in his private and political life. Whether they also pose a fatal threat to his Presidency is far from clear, however, and the evidence in both cases is unrelated to the Russian collusion claims that set these prosecutions in motion.

A Virginia jury convicted Mr. Manafort on eight counts of fraud in his personal financial dealings, while failing to reach a unanimous...