And she's out of the Democratic party now.

The seat belongs to the individual on the ballot, it doesn't belong to the party. That means someone holding a seat can go against their party which is generally good as it prevents the party from having too much power. But the downside is when you get a lunatic in a seat and all you can do is get rid of them in the next election.

Which is what's happening. Sinema was going to be challenged in the primary and likely to lose. She decided to go independent, but last I heard, after seeing her polling numbers she's dropped out completely.

As I said elsewhere, the Democratic Party is the 42/50 party while the GOP is the 0/50 party. You probably shouldn't vote for a party no matter who's elected, but even if you took this naive approach, the Dems might potentially become a 50/57 party. The GOP will always be the 0/50 party.

Ideally you'd check the platforms of every candidate at the primary stage through to the general and vote accordingly. If most people did that the Dems could be a 50/50 party.

But most people aren't that involved politically. Blue no matter know, while isn't ideal, is still better than voting GOP or not voting at all.