Our rankings are based on the Real Clear Politics averages of national polls and those in the states of South Carolina and Nevada. We also factored in the candidates' delegate count and their finish in Iowa and New Hampshire, as well their momentum (or lack thereof) over the past few weeks.
Since New Hampshire, two more Republican candidates have dropped out of the race: former Hewlett-Packard CEO Carly Fiorina and New Jersey Gov. Chris Christie.
Here's a look at where all the candidates stand.
All poll results as of Thursday.
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His national and state poll numbers continue to plunge, and it's not clear where he can be competitive from here on out.
National polling average among Republican voters: 6.6% (5th)
South Carolina: 6.8% (6th)
Nevada: 6% (5th)
Last month: 9
(AP Photo/Paul Sancya)
7. Jeb Bush, Republican, former Florida governor
Bush, once viewed as the clear front-runner, has seen Trump sap the momentum he had built after his official campaign announcement in June.
His poll numbers have slumped across the board — his 17% national average in July has dipped almost 12 points over the past seven months, even as other candidates have dropped out.
Bush finished in a strong fourth place in New Hampshire, ahead of Rubio. He has brought on his brother, former President George W. Bush, to campaign for him in South Carolina, where observers say he needs another strong finish to build momentum as a possible establishment alternative to Trump.
Bush has showed, too, that he is a dynamic fund-raiser. And he retains significant resources, especially in the super PAC allied with his campaign.
National polling average among Republican voters: 5.4% (6th)
South Carolina: 10.4% (4th)
Nevada: 3% (6th)
Last month: 6
(AP Photo/Andrew Harnik)
6. John Kasich, Republican, Ohio governor
Kasich finished an impressive second in New Hampshire, a feat that has proven to be a serious boon to his presidential ambitions. But South Carolina and subsequent states present potential sumbling blocks.
Those who talk up Kasich say he is a successful governor of a swing state with a record to point to and clear bipartisan appeal. He also has a plethora of experience from serving nearly two decades in Congress, including foreign-policy areas and his time as chair of the US House budget committee.
But that same bipartisan brand could hurt Kasich with the GOP base beyond New Hampshire. He is to the left of most GOP candidates on immigration reform, and he expanded the federal Medicaid program under the Affordable Care Act — two issues that could doom him with hard-line conservatives.
National polling average among Republican voters: 8.6% (4th)
South Carolina: 9.6% (5th)
Nevada: 7% (4th)
Last month: 7
(AP Photo/Carlos Osorio)
5. Marco Rubio, Republican, senator from Florida
Rubio was perhaps the biggest winner from the Iowa caucuses, as he and his campaign played the "expectations game" perfectly and surprised with a stronger-than-expected, third-place finish right behind Trump.
He stumbled ahead of New Hampshire with a widely panned debate performance, ending up fifth in the primary. Now, he has seen a swing of momentum back his way ahead of the South Carolina primary. On Wednesday, he secured the endorsement of South Carolina Gov. Nikki Haley, who is highly popular in the state.
And his polling trends leading into primary day look very much like they did heading into the Iowa caucuses.
National polling average among Republican voters: 16% (3rd)
South Carolina: 17.1% (3rd)
Nevada: 19% (3rd)
Last month: 4
(Photo by Alex Wong/Getty Images)
4. Bernie Sanders, Democrat, senator from Vermont
Sanders scored not only a major upset over Clinton in New Hampshire, but also a gigantic, 22-point victory — a feat that was unthinkable even last summer.
And the door to the nomination that was long thought closed has creaked even more open.
He is the big mover in our rankings.
Sanders appears to be peaking at the right time. His momentum, and the grassroots support and donations behind it, has evoked comparisons to the 2008 rise of Barack Obama, then an Illinois senator.
But Sanders still faces daunting challenges against the behemoth that is Clinton and her campaign. There are questions about whether he's a legitimate threat in the long haul. A win in Nevada — a state where the electorate contains a much higher percentage of minorities — would dispel the notion of Clinton's perceived "firewall" in states like Nevada and South Carolina.
National polling average among Democratic voters: 42% (2nd)
South Carolina: 33.6% (2nd)
Nevada: 46.3% (2nd)
Last month: 5
(AP Photo/Evan Vucci)
3. Ted Cruz, Republican, senator from Texas
Cruz rode a record turnout in Iowa — something that was supposed to benefit Trump — to a surprisingly robust victory. He has built momentum heading into his target — the "SEC primary" states in the South, many of which vote March 1.
Cruz's under-the-radar campaign has put him in sneakily good position to capture the nomination. And his eye-popping fund-raising numbers mean he will most likely be in the race for the long haul.
Cruz inspires a flood of enthusiasm among the GOP base, and he may be the best-positioned candidate from within the political sphere to back up the notion that he's not a typical politician, that he is the outsider the base wants despite his day job in Washington.
National polling average among Republican voters: 20.6% (2nd)
South Carolina: 17.6% (2nd)
Nevada: 20% (2nd)
Last month: 3
(Photo by Sean Rayford/Getty Images)
2. Donald Trump, Republican, businessman
Trump has lit the political world on fire since his entry into the race last summer. And he has showed surprising staying power — we're now on month No. 9 of the Trump show.
He lost in Iowa. But the Hawkeye State was never fertile ground for someone like Trump, a brash New York billionaire. And a major win in the Granite State proved the formidable nature of his candidacy.
There's a clear appetite among Republican primary voters for someone like Trump, who entered the race to controversy surrounding his position on illegal immigration. Business Insider discovered more of that when we followed him on the trail for a week last year.
Trump, leading big in polls of South Carolina, could start charting his course toward the nomination in the Palmetto State.
National polling average among Republican voters: 34.2% (1st)
South Carolina: 33.5% (1st)
Nevada: 42% (1st)
Last month: 2
(Photo by Jabin Botsford/The Washington Post via Getty Images)
1. Hillary Clinton, Democrat, former secretary of state
U.S. Democratic presidential candidate Hillary Clinton speaks during a campaign rally at the International Union of Painters and Allied Trades (IUPAT) training center in Henderson
Clinton is (an increasingly shaky) No. 1 here because she has proved formidable in polling and fund-raising and has still has a clearer path to the nomination than anyone on the GOP side.
But Clinton is facing an increasingly rugged challenge from Sanders, who threatens to win easily a state she captured in 2008. Her campaign is teetering perhaps more than it ever has been, as her "firewall" — her strength in states with more diverse Democratic electorates — is showing signs of melting in Nevada.
If Clinton loses in Nevada, it would be a major blow to a campaign that once seemed to boast the inevitable Democratic presidential nominee.
National polling average among Democratic voters: 46.3% (1st)
South Carolina: 56.7% (1st)
Nevada: 48.7% (1st)
Last month: 1
(Photo by: Virginia Sherwood/MSNBC/NBCU Photo Bank via Getty Images)
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And to the polls: Here's a look at where the candidates stand in their respective parties when combining their delegate totals and their standing in national, South Carolina, and Nevada polls.