How so? If you're referring to the open primary system, that doesn't mean there are no primary challengers. In a sense, it means everyone but the incumbent is a primary challenger... but more meaningfully the distinction of a primary challenger is that they belong to the same party as the incumbent, which the open primary system still admits. It might actually help: as I've pointed out before, Feinstein got well more than 2x (actually, almost 4x) any challenger in 2012 primary so democrats could have guaranteed themselves the general election if Feinstein voters had all picked a single other Democrat as an alternate and flipped a coin (... or if even half of them did so).
> How so? If you're referring to the open primary system, that doesn't mean there are no primary challengers. In a sense, it means everyone but the incumbent is a primary challenger
It means "primary challenge" (which refers to an intra-party run in a partisan primary with a segregated pool of cnadidates) is a meaningless distinction when running against someone, since any time you run for an office, it means running in the general, non-partisan pool of candidates in the "primary" election.
I think it's clear that what was meant was "You'd have to be a Democrat to have much access to many of the votes that have been going to Feinstein, and you'd need access to those to win". Again, same party as the incumbent being the salient feature. If you want to assert it no longer quite fits, I'm on board; I don't agree that it's fair to call it "a meaningless phrase".
Given that its California, "primary challenge" is a meaningless phrase for anything but a Presidential race.