He was trailing Allen by 4 pts that's why he didn't run in 2006. Mark Warner will be a tough challenger, but it won't be that easy. And voters look at the top of the ticket, if Hillary Clinton is the nominee, her liberal voting record will have an effect on the race. Just like John Kerry's did on the senate candidates as well.
Thats not always the case. John Kerry hurt Dems in open seats that they were defending in deep South states that Kerry lost by at least 15 points save for Florida. Every Democratic Senate incumbent was reelected in 2004 except for Tom Daschle. Ken Salazar was able to win an open seat in Colorado even as Kerry was losing the state by three points. Virginia would likely be similar if Mark Warner ran.