No sooner do I write a column about “vanishing Democratic moderates,” pegged on the retirement announcements of Sen. Joseph Lieberman and Rep. Jane Harman, than another one announces he’s not running again, Sen. Jim Webb. And I could have added that Sen. Kent Conrad, D-N.D., announced his retirement even earlier. We’re beginning to see a pattern here, as we did in the 2010 election cycle: Democrats leaving what they have reason to believe is a sinking ship.