Big news in the Senate today, with Ben Nelson, a Democrat from Nebraska, announcing his retirement.

Nelson has consistently been one of the most conservative Democrats in the Senate, and during the very brief period in 2009 in which the caucus held 60 seats, his was usually the toughest vote to secure.

Still, no one should think that he is interchangeable with the mainstream Republican who will likely replace him; Nelson, after all, did eventually vote for the Affordable Care Act, the stimulus bill and numerous other Democratic priorities.

The Democrats’ hopes of holding the seat come down, at this point, to the vague possibility that former Nebraska senator and governor Bob Kerrey will attempt a comeback. Even if Democrats can talk him into running — and when in office, Kerrey was never much of a team player — it’s a pretty tenuous effort at best. Kerrey is 68 and retired from the Senate in 2000 (when he was replaced, interestingly enough, by Ben Nelson). Since then, he has spent most of his time as president of the New School in New York City.

Kerrey was always popular in Nebraska, and he has been rumored before to be interested in a comeback, but there is no guarantee that voters who haven’t seen him on a ballot since 1994 will be very interested in him.

The state of play in the Senate before the Benator’s retirement favored Republicans but not decisively. According to Charlie Cook’s latest ratings, no seat was favored to switch parties, but the Democrats had a much tougher hand to play, with eight current Democratic seats rated as a “toss up” and two more leaning Dem – compared to only two Republican seats really in play.

Still, with 53 current senators, Democrats did have some margin of error. But with Nebraska presumably moving soon from toss-up to likely, or even safe, Republican, that margin of error is dramatically decreased. While a lot of what you’ll see today is how little liberals will miss Nelson, the truth is this is very good news for the GOP.