1. Marco Rubio. Trails badly in current polls, ranking seventh at 5 percent, but high upside potential.

Assets: Foreign policy looms uncharacteristically large in the current cycle, and Rubio is the most knowledgeable and fluent current contender on everything from Russia to Cuba to the Middle East. The son of Cuban immigrants, he can break into flawless Spanish (so can Jeb Bush) and speak passionately about the American story in a party that lost the Hispanic vote by 44 points in 2012.

Liabilities (in the primaries): His Gang of Eight immigration apostasy, though his current enforcement-first position has wide appeal. Second, after Barack Obama, will voters want another first-term senator with no executive experience? (Same for Cruz and Paul.)

Major appeal: Fresh, young, dynamic persona is a powerful counterpoint to Clinton fatigue.