What's the most vulnerable Senate seat in the country?

The open seat in Colorado. That's nothing Coloradolib readers didn't already know. But for posterity, from The Washington Post:

Allard's announcement jumps this race up to the top of Democrats' opportunity list. Despite the chatter about a candidacy by Denver Mayor John Hickenlooper, Rep. Mark Udall should have the Democratic field to himself. For Republicans, former Rep. Scott McInnis is in the race, and his former House colleague, Bob Schaffer, is actively considering it. If those two men comprise the top tier of GOP primary candidates, expect a nasty fight between fiscal conservatives (McInnis) and social conservatives (Schaffer) within the Colorado Republican Party.