Let's say you're 42 years old—a spring chicken, and yet already a high-profile member of Congress. You've got a seat in the Senate that seems fairly secure. Meanwhile, the Republican presidential field is jam-packed, one of the strongest groups in recent memory, and running for president would mean surrendering your Senate seat. Your whole career is ahead of you. Why run for president?
Only Marco Rubio can really answer that question, and the Florida senator is unlikely to answer it in those terms. But he's made his choice. He told donors he was running on Monday morning, and he'll kick off his campaign with an appearance in Miami in the early evening.
Here's one theory, though: Rubio is the perfect second choice for GOP voters. Though he isn't leading in polls, many Republicans say they're willing to consider him and very few rule him out, compared to most of his rivals. All he needs is for other candidates who inspire more passion to falter and their supporters to coalesce around him, and he could be set to take on Hillary Clinton.
Rubio's support right now is decent but not stunning—in RealClearPolitics's average, he's at 7.3 percent, better than much of the field but behind Jeb Bush, Scott Walker, Ted Cruz, Rand Paul, Ben Carson, and Mike Huckabee. (Cruz and Paul have both benefited from recent leaps after declaring their candidacies.) Many of these candidates have extremely motivated supporters, but each of them has apparent limitations. Bush's record is more moderate than many Republican primary voters, and he's taken flak for his stances on Common Core and immigration. Walker has never run a national campaign and is under fire for flip-flopping. Cruz and Paul are both far enough outside of the political mainstream that they'd likely have to cobble together unprecedented coalitions to win. Carson is still largely a cipher and has never run a race. Huckabee's culture-warrior persona seems to limit him.