Sen. Bill Nelson (D-Fla) beats all potential challengers in a new poll. (J. Scott Applewhite/AP)

Like President Obama, Sen. Bill Nelson (D) gets mediocre approval ratings, but he’s benefiting from a weak field of challengers.

The Sachs/Mason-Dixon poll of 625 registered voters found Nelson leading every potential challenger by double-digits — except for West, who announced earlier this week that he would not run.

Over half of Republicans are still undecided on a candidate, and no declared contender cracks 10 percent. The most popular choice is Rep. Vern Buchanan (R-Fla.), who is still undecided on a bid. Former state Rep. Adam Hasner and former Sen. George LeMieux are at 8 and 7 percent, respectively.

The establishment Republican candidate was state Senate President Mike Haridopolos, who dropped out unexpectedly in July. Party faithful have been gravitating to Hasner, leading to speculation that former governor Jeb Bush will endorse soon.

Retired Army Col. Mike McCalister gets 2 percent in the Mason-Dixon poll after taking 15 percent in a Quinnipiac survey at the beginning of the month, a sign of how fickle early polls can be.

Hasner backers points out that his numbers are rising while LeMieux’s fall.

“It demonstrates one thing very clearly: Adam Hasner is the most competitive Republican in the race against Bill Nelson and is steadily building a foundation to defeat him,” said Hasner strategist Rick Wilson.

But some Republicans are openly pining for a new candidate.

“They’re all good individuals, but I don’t see anybody setting the world on fire,” state party treasurer Jeff Howell said last week. After he dropped out, Haridopolos declared that “it is going to take a Jeb Bush or an Allen West” to defeat Nelson. Supporters of Hasner say it’s just sour grapes.

McCalister and LeMieux have started ganging up on Hasner, “It appears that the Republican field is going to get a lot bloodier before it gets better,” said Democratic Senatorial Campaign Committee communications director Matt Canter.

This far out, it’s not surprising that most Republicans are undecided. Nelson is still under 50 percent in all these matchups, not a great place for an incumbent. (Democrats point out that Sen. Marco Rubio (R-Fla.) is below 50 percent favorability in polls right now, too.) No matter what, Nelson is vulnerable.

In fact, it’s surprising how many voters are not sure how they feel about Nelson given that he’s been in the Senate for 11 years and in politics for decades. Republicans argue that he’s never been defined by a tough fight. They just don’t agree on who can make that happen.