Democratic frontrunner Hillary Clinton may have the aura of inevitability about her, but it is another world for the GOP, where social conservatives and fiscal hawks are galloping alongside moderates as they race to see which philosophy prevails.
At no time in the last half century has such a diverse crop of politicians sought a presidential nomination.
Republican tradition has largely had it that party leaders engage in the so-called "imaginary primary," in which the establishment anoints a consensus candidate before the primary contests kick off at the start of an election year.
Democrats engage in such formulas too, and Clinton, a former first lady, senator and secretary of state under President Barack Obama, has clearly emerged as her party's likely flag-bearer.
But "there is no consensus candidate" for Republicans in 2016, Merle Black, a political science professor at Emory University, told AFP.
Instead there are governors like Scott Walker of Wisconsin and first-term senators Ted Cruz and Marco Rubio, two Cuban-Americans with rockstar-like followings.
Then there is former Hewlett-Packard chief Carly Fiorina and neurosurgeon and political neophyte Ben Carson.
And don't forget ex-governor and Baptist minister Mike Huckabee, New York's pro-environment 9/11 governor George Pataki, and another Bush for good measure.
Even Donald Trump, the property tycoon and media figure who perpetually threatens to run, is in the mix.
"They're having a thorough vetting - not just of people and candidates, but also of issues and the direction of the party," University of New Hampshire political science professor Dante Scala told AFP.
Having not held the White House since George W Bush is "a good thing for a party to do to renew itself," he said.
There has been no Republican commander-in-chief other than Bushes since the 1980s, and candidates are all-too-aware of the historic difficulty in a party holding the White House for three straight terms.
Wanted: Credible messenger
With the GOP direction in flux, Republicans are "fishing for a candidate," explained Tim Malloy, assistant director of Quinnipiac University Polling Institute.
A Quinnipiac poll released Thursday shows five candidates - Walker, Rubio, Huckabee, Carson and former Florida governor Jeb Bush - bunched at the top, each with 10 per cent support.
"When you see that Jeb Bush did not gain a huge amount of traction early on, when you see that Rubio came out hot and then has kind of leveled off, a lot of candidates who think they have potential are probably looking and saying 'No one's got this yet,'" Malloy said.
According to Black, the GOP's deep bench includes a "new generation of Republican politicians," like Senator Rand Paul whose profile has risen with his opposition to the government surveillance program that gathers data on millions of Americans.
It also includes politicians like Rick Santorum and Rick Perry who ran in 2012 and "want a second shot at it," he said.
"There are lots of issues about the direction of the Republican Party and how viable the party will be in the future, and there are lots of different ideas," Black added.
"That's what primaries are about, to see who can put together a message, and who can also appear to be a credible messenger."
A lot also rests on who can raise the necessary money. With finance laws easing dramatically in 2010, sewing up a steady funding stream is undeniably crucial.
"If a candidate can find an angel or two, they can get in and at least be credible in the short-run," said Linda Fowler, a professor of government at Dartmouth College.
Elite donors and activists often "seem to value ideological purity over experience," said Fowler, perhaps explaining the large number of talented but ultimately unseasoned candidates.
The Republican field may well whittle down in August with the start of televised debates, where organizers are under pressure to find ways to accommodate large numbers of candidates on the same stage.
Some, Black said, will "fail embarrassingly" by the time Iowa, New Hampshire and South Carolina hold their early primary contests.
Fowler concedes that despite clear differences in experience, popularity and viability, it remains too early to write off any of the candidates.
"Half will be out by September," she predicted of the Republican hopefuls. "I just don't know which half."
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