Bush likeliest to lead Republican challenge
For now, Jeb Bush (62), the former Florida governor, is the closest the Republicans have to an establishment candidate.
As the brother and son of former presidents, Mr Bush will have plenty of money and name recognition.
But he faces numerous challenges from a conservative base that sees him as too moderate - he supports controversial policy positions on education and immigration reform that are anathema to them.
Experts say the issue of dynasty could also be a problem for Mr Bush, who is perceived as a hand-picked successor to the Bush dynasty. That is to say, he is not a shoo-in.
In past electoral cycles, multiple "not ready for prime-time" Tea Party candidates split the conservative vote. Now, the conservative candidates vying for the nomination are much more legitimate.
No one should underestimate Ted Cruz. The fiery Republican senator from Texas (44) launched his presidential campaign with a deeply religious speech at the world's largest evangelical Christian university.
He is incredibly intelligent and reflects the identity of the conservative base better than any other candidate.
He is adored by the Tea Party for stridently opposing President Barack Obama on issues including guns, abortion, immigration to health care.
There is also Rand Paul (52), the charismatic libertarian Kentucky senator with youth appeal. He hopes to stand out from his party rivals with an unorthodox platform that appeals to the young and mixes traditional conservative positions with some more progressive stances.
But his anti-interventionist foreign policy has not aged well since Russia invaded Crimea and Islamic State started decapitating people.
He is also harmed by comparison to Mr Cruz, who is widely seen as a better, more mainstream, version of him.
There are also several hybrid candidates - people who could potentially be acceptable to both grassroots conservatives and the establishment - that threaten Mr Bush's hold on establishment support. Senator Marco Rubio, of Florida, the son of a bartender who emigrated from Cuba, boasts an "American dream" story that is nothing short of inspiring.
If the goal is to elect a Republican who can articulate to non-conservatives why conservatism is the best philosophy, he is the man. But Mr Rubio (43) is still haunted by his support of immigration reform, which many in his base vehemently oppose.
There is also Scott Walker (47, left), the Wisconsin governor, who took on the unions in his home state and won. That was impressive. He is considered a front-runner, but may not be ready for prime time.