They are the last line of defense. Which means when they fail, election subversion becomes encoded as a matter of law. First in that state, and then federally, as that state manipulates its rules to send legislators and Presidential electors according to their partisan whims.
When the Supreme Court decided that gerrymandering was a state-by-state matter, it meant that some states were given the go-ahead to gerrymander. No requirement "free and open elections" makes it illegal to put all of your opponents in a single voting district. It's obviously unfair, but freedom from being "obviously unfair" is not explicitly given in the Constitution and therefore not a right any of us have.
When the Supreme Court decided that gerrymandering was a state-by-state matter, it meant that some states were given the go-ahead to gerrymander. No requirement "free and open elections" makes it illegal to put all of your opponents in a single voting district. It's obviously unfair, but freedom from being "obviously unfair" is not explicitly given in the Constitution and therefore not a right any of us have.