Florida Sen. Marco Rubio’s campaign, which officially kicks off Monday, has so far attracted paltry support from Republican voters, according to polls in Iowa and New Hampshire, as well as nationally. He’s down near Chris Christie! Yet, when we talk about him in the FiveThirtyEight office, we usually put Rubio in the top tier, in front of everyone except Jeb Bush and Scott Walker, the two candidates at the top of the polls. Why? Rubio is both electable and conservative, and in optimal proportions. He’s in a position to satisfy the GOP establishment, tea party-aligned voters and social conservatives. In...