1. Most prosecutors are liberals. It's not like right wing law students go looking for jobs in prosecutor's offices. I can't think of any from my class way back when.
2. The problem is more that judges and prosecutors are old colleagues. The slant is that they all know the defendants are guilty. There's so much crime, and aversion to losing trials, that (unless politics is involved) prosecutors tend to only bring cases where the defendant is clearly guilty. They like shooting fish in a barrel. If they find that there are problems with a case they tend to let the defendant get continuance after continuance until the case just goes away on its own.