Ryan: Budget axe cuts both ways — catnip to conservatives but people want their Medicare. Carries stigma of 2012 election loss as running mate. Tea party not happy with his late 2013 budget deal. Comments in March about cultural "tailspin" in inner cities struck some as veiled racism. Deflection: Called his remark "inarticulate."
Santorum: Overshadowed by newer conservative figures. Deflection: Being overshadowed means being an underdog, and he can thrive at that. Feisty 2012 campaign became the biggest threat to Romney's march to the nomination. New book contains provocative passages for future rivals to dredge up.
Walker: Some things that give him huge appeal with GOP conservatives — taking on unions, most notably — would whip up Democratic critics in general election. Wisconsin has lagged in job creation. Release of emails in February shed light on criminal investigation into whether Walker's aides were illegally doing campaign work for the 2010 governor's election while being paid as county employees. Walker, then a county executive, wasn't charged but the episode has proved a distraction.
RUN SHADOW CAMPAIGN: One way to run without running is to have a political action committee to promote ideas or other candidates for office, or to hire advisers who can switch to a campaign when the time comes.
Biden: Constrained by his current job, but tapped longtime adviser and former lobbyist Steve Ricchetti to be his new chief of staff; maintains close contact with political advisers past and present.
Clinton: Ready for Hillary super PAC set up by supporters is laying groundwork, so are others. Several old Clinton hands are advising the group, including Craig T. Smith and Harold Ickes.
Cuomo: Overshadowed by Clinton's shadow campaign. Considered a likely contender if Clinton ends up not running.
O'Malley: Set up a PAC called O'Say Can You See and hired two people for fundraising and communications.