It’s called a “faithless elector” and what happens depends on the law of the state the elector is representing. Some states void the vote without penalty, some void it with some penalty, some allow the vote but with penalty, some allow the vote with no penalty, and some have no law at all (which seems like no difference from allowing with no penalty).
It’s entirely conceivable that enough faithless electors from states that do not void the vote could swing an election, though there’s never been enough to do so before.
It’s populism. You have to see if someone’s policy positions are consistent over time, that they have a specific ideal they are following, vs what they think is the most popular policies.
Granted, it’s a huge pain with new politicians when they don’t have that history, but I think Tulsi had a pretty clear history that showed she wasn’t progressive.