Nelson's races always appear to be close in the beginning, then he wins by a million votes.
To be fair, he's gotten very lucky both with the national environment and with attracting seemingly-strong but actually quite weak opponents.
Very true. A strong candidate like Tom Rooney or Jeff Atwater would beat him.
Not to mention his last two elections were Democrat waves: 2006, 2012
Connie Mack IV is the perfect example of a "seemingly strong but actually weak" opponent. On paper he was the perfect candidate, but in reality he turned out to be deeply flawed and incompetent. His 2006 opponent (in an even bigger wave than 2012), Katherine Harris was also a very controversial figure, while popular with her base she was deeply unpopular with many swing voters and Democrats in the anti-Bush year of 2006 due to her role in the controversial recount. In addition to the two I first mentioned, David Jolly and Carlos Curbelo are also good candidates (if they win reelection).