No one can question whether Sen. Bill Nelson, D-Fla., a 74-year-old former astronaut, has the stamina to run for a fourth term. After all, Nelson was caught on tape in 2015 doing 46 pushups after losing a bet on a hockey game with Sen. Dick Durbin, D-Ill. ("Thank God you lost and I didn't!" Durbin exclaimed.) More recently, he put Sen. Cory Booker, D-N.J., decades his junior, to shame with a similar feat.

But a new poll from the University of North Florida suggests that Nelson will have a real fight on his hands if Republican Gov. Rick Scott takes him on next fall. Scott has expressed interest, and President Trump, whom he embraced immediately after the Florida primary, has encouraged him to get in.

The good news for Nelson in this poll is that he leads Scott by six points and has a positive job approval rating at 42 to 28 percent. The bad news is that he clocks into the head-to-head matchup with just 44 percent support — a bit weak for a three-term incumbent senator who hasn't faced a serious challenge since he was first elected 17 years ago. And that's certainly below the threshold where any incumbent can be considered "safe."

As for Scott's mere 38 percent support, that's also worth keeping in mind. But Scott, a former healthcare business executive who has a personal fortune to spend on his own campaign, did not find his own extreme unpopularity an obstacle to winning re-election in 2014 against former Gov. (and now Tampa-area Congressman) Charlie Crist.

In two polls taken in March 2013 — that election cycle's equivalent to this month — Scott trailed Crist by 12 and 16 points. Scott led in only one of the last 13 polls taken in October and November of the election year. But after spending about $100 million of his own money, he still beat Crist by just over one percentage point. So don't count him out over bad numbers alone.

A note on the poll, though: It was conducted over a two-week period in late February. That's a bit odd, and out of keeping with the standard practice of polling over three or four days, but that might not matter so much this early in the contest when no one is campaigning and Scott hasn't even declared.

Another somewhat related note on Florida: For more than three decades leading up to 2006, Republicans in the state had been gaining ground against Democrats in the category of partisan voter registration. The Republicans' momentum collapsed in 2008, along with the party's fortunes, and GOP registration fell from about 93 percent to about 86 of the Democrats' total. Only this year, using the new numbers as of December 31, have they finally regained all of the ground they had lost at the beginning of the Obama era, bringing them to within 300,000 registered voters of party parity.