Former Secretary of State Hillary Clinton is heading back to Iowa for the first time since 2008 this weekend to speak at the 37th Harkin Steak Fry. And since she says she'll probably decide on a 2016 run "after the first of the year," ABC News took a closer look at three other Democrats who could challenge her for the nomination if she decides to run for the White House for the second time.

1. Sen. Elizabeth Warren

The Massachusetts senator - speaking to ABC News' David Muir earlier this year - said she is not running for president, but could she change her mind? It's happened with other candidates before, including then-Sen. Barack Obama before the 2008 election. Warren, who recently released a book, is a favorite among more progressive Democrats. But an ABC News/Washington Post poll in June showed that among registered voters, 69 percent of Democrats and Democratic-leaning independents favor Clinton for the party's nomination over seven other hypothetical contenders. Warren received only 7 percent.

(ABC News)

2. Vice President Joe Biden

As the current vice president, Biden could not be any closer to the Oval Office than he is in his current position. The former Delaware senator, now in his 70s, has not ruled out another run for the White House.

He's already run twice for the highest elected office in the land. In fact, just days after his potential 2016 rival Clinton visits Iowa, the vice president will be making his own stop in the Hawkeye State, the Des Moines Register reported. The same poll in June that showed a majority of registered Democrats and Democratic-leaning independents favor Clinton by a large margin showed Joe Biden garnering 12 percent support.

(Susan Walsh/AP Photo)

3. Gov. Martin O'Malley

The governor of Maryland told Fusion's Jorge Ramos last month that he is "seriously considering running in 2016," and there are indications he might run regardless of whether Clinton makes a run or not. While he may not be as well-known nationally as Warren, Biden or Clinton, O'Malley has made news recently by publicly distancing himself from the White House over immigration. O'Malley was also just in Iowa, stopping there on Sunday to campaign for Jack Hatch, a state lawmaker challenging incumbent Republican Terry Branstad for governor.

(Larry French/Getty Images)

ABC News' Shushannah Walshe contributed to this report.

For more on the 2016 race for president, tune in Sunday to ABC News' "This Week."

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