John Edwards who ran for President in 2008, but dropped out because it was discovered he had a baby mama/mistress, and paid her off with campaign money. Edwards was charged with 6 counts of violating various federal campaign laws in 2011. He was found guilty of one, and because the jury couldn't agree on the other five charges, the Judge called a mistrial. The DOJ chose not to re-try.
I'd say the DOJ wasn't interested in winning this case. They gave up without a fight. Obama was in the White House, and Holder was Attorney General.
President Trump would be facing a hostile New York court system.
-PJ
And Edwards was a Democrat.