A new Mason-Dixon poll, reported by Politico, shows incumbent Florida Democrat Bill Nelson in danger of losing his Senate seat in 2012 — particularly against one challenger.  Nelson, serving his second term, falls short of 50% against all challengers in the poll, even the least known potential Republican candidates, a bad sign for an incumbent.  When compared to Jeb Bush, though, Nelson falls to 41%:

“Regardless of his many years of public service, Sen. Nelson’s reelection in 2012 is anything but a sure thing – in fact, any of his prospective Republican challengers is positioned to be competitive and possibly win,” said Ron Sachs, president and CEO of Ron Sachs Communications who commissioned the poll.  “This contest will be a national story, as will the fact that, if he ran and the  election were held today, former Gov. Jeb Bush would beat Sen. Nelson.”

In the poll of 625 registered Florida voters conducted Feb 9-10, 2011 by MasonDixon Polling & Research, Sen. Nelson fails to gain 50 percent support against any potential Republican challenger.

“Bill Nelson will clearly have a much more difficult path to victory next year than he had in 2006. He is in trouble if voters in 2012 are looking to throw out career politicians like they did in the past election,” said Mason-Dixon Managing Director Brad Coker. “The fact that Sen. Nelson currently can’t crack 50 percent against any GOP challenger in a head-to-head match-up, including several lesser-known lawmakers, is a clear sign of vulnerability.”

A survey released yesterday showed that Jeb Bush would lose to Barack Obama by over 2o points in a presidential race, making Bush one of the least desirable challengers to Obama.  Bush doesn’t have that problem at home in Florida.  He retired with high levels of job approval (in the 60s) and still wields potent political force.  When Bush broke from Charlie Crist last year over his veto on an education bill, it was devastating enough that Crist left the party within two weeks.

But it isn’t just Bush within striking distance of Nelson, either.  Rep. Connie Mack starts off at a 45-40 disadvantage, almost within the margin of error.  George LeMieux is much farther back at 49/35, but Nelson can’t get to 50% against him, either.  The same is true for Florida Senate President Mike Haridopolos, who starts off almost getting doubled at 48/25 but still keeps Nelson from a majority.

Not only does Nelson fail to score 50%, but the poll sample may also be a little friendly to him, too.  It gives Democrats a four-point advantage, 44/40/16, but the exit polling from the midterms showed a different balance of 36/36/29 (rounding artifacts give us 101%).  That may balance out a little as Nelson does better with independents against all challengers; he still loses to Bush among unaffiliateds, but by a narrower margin, 42/45.

These are not re-elect numbers for Nelson, although he’s not doing as badly as his colleague Ben Nelson of Nebraska.  If Jeb jumps into the race, and that seems like a good fit, Nelson will be in serious trouble.