Who would they appoint as judges...it is the litmus test
Bingo
It extends far beyond the candidate themselves. Here in Mississippi we had a blue dog democrat named Gene Taylor that was our representative for years. He was pretty conservative, certainly much more so than any democrat and most republicans from the northeastern states. On the face of his voting record and what he stood for he was actually OK, most conservative republicans would be pretty happy with him. The problem wasn't Gene himself, it's that he was a democrat and that meant that when it came to all the political maneuvering in Congress he had to side with the democrats. When it came to electing the speaker of the house he voted for Nancy Pelosi, even though I'm sure he had to hold his nose while doing that. If he didn't he'd be ostracized by the party and lose all his committee assignments which is where the power is held. Same things with Obama's appointments, he'd have to vote for them or risk alienating the democratic party. It was a damned if you do, damned if you don't situations. He really should have switched to the republican party and he'd probably have held on to his seat, but he didn't and now he's gone.
It's not all the person themselves. The mere fact that you run on a democratic ticket means that you're beholden to the democrat party and you have to cave to their desires. Webb might not be that bad, but when it comes to getting legislation through congress he's going to have to cave to democratic leaders in order to get anything done. Same thing with court appointments, he'll have to nominate committed leftists like Kagan, Ginsberg, and Sotomayer to be able to get them through the senate appointment. If his own party leaders won't support his nominees then they'll be shut down, he'll have to go ultra leftist to get anything done.
ANYONE with a D next to their name is a non-starter, you're not electing a person you're electing a party and democrats today are committed socialists, end of story. Most republicans are bad enough, a democrat of any sort is a non-starter.