The party whose SCOTUS justices actually stole a presidential election in such a nakedly partisan way that David Souter didn't even want to bother serving along with those cretins anymore.
In terms of quality, the worst Supreme Court decision ever was indeed Bush v. Gore (although that decision did not actually steal the election from Gore, it only stole from Gore his right to find out whether he actually won or lost). But in terms of quantity, it's the liberal wing of the Supreme Court that has rendered far more activist decisions in the last 60 years. So I blame Supreme Court Justices of all ideological stripes, appointed by Presidents of both parties, for gradually destroying democracy in this country.
Nixon, Ford, Reagan, both the Bushes, and even Trump, all nominated justices that displeased the conservative base at one point or another. Earl Warren, who oversaw the greatest expansion of civil rights and liberty protections, was a Republican. Byron White, who dissented in
Roe, was appointed by JFK.
Both the Rehnquist and Roberts Court have appeased liberals on occasion to the shock of conservatives, and frankly the only
true ideologues on the Supreme Court are Thomas and Alito. But we have also seen cases such as the one (whose name escapes me) which saw Thomas concur with the liberals on police surveillance and Breyer (who often sides with prosecutors and law enforcement) dissenting.
Democratic presidents will predictably appoint liberal-leaning judges just as Republicans will appoint conservatives, but the Court as of late has erred on the side of caution and compromise in order to preserve its legitimacy. This, at least, is John Roberts' main concern. And while that may be a political strategy, it is not for any ideological cause.