Corruption makes a lot of money for politicians on both sides of the aisle in America.

Democrats didn't go after Republican corruption because they too would be caught in the net if it was indeed done via an independent Justice System.

You see this exact same effect in other countries with voting systems that boost the representation of the two largest parties - neither of them ever goes after Corruption because both of them have been thoroughly captured by corrupt politicians and you even get the effect in Parliamentary systems that some of the few things they both vote together on is weakening existing anti-corruption legislation and defeating new anti-corruption legislation.

Parties which regularly get real power, such as the two main parties in Power Duopoly "democracies", always attract the most crooked people around (similarly to power monopolies - I.e autocracies) and are guaranted to turn rotten even when they start from the most honest and fair beginnings.