Hillary Clinton's potential challengers

Updated 7:52 PM ET, Mon February 23, 2015
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Sen. Elizabeth Warren of Massachusetts has repeatedly said she's not running for president, but the Democrat -- a favorite among progressives -- has been urged to run by populists in her party. A first-term senator elected in 2012, Warren is known for her attempts to take on Wall Street -- both as a senator and as the creator of the Consumer Financial Protection Bureau during President Obama's first term. Chip Somodevilla/Getty Images
Maryland Gov. Martin O'Malley has been laying the groundwork for a potential presidential campaign, headlining 80 fundraisers and 40 campaign events around the country. A two-term governor, his time in office ends in January. The Democrat's resume as governor includes legalizing same-sex marriage, repealing the death penalty and enacting strict gun-control laws. Mark Makela/Getty Images
Vice President Joe Biden and Hillary Clinton faced off in the early 2008 Democratic primaries, and both are weighing 2016 bids. A 2016 run would mark Biden's third bid for president; he also ran in 1988. Before becoming vice president, Biden served as a U.S. senator from Delaware for 36 years. He has already made visits to early primary states this cycle on official White House business. And if Clinton runs, Biden said her decision wouldn't affect his own. Andrew Gombert/Pool/Getty Images
U.S. Sen. Bernie Sanders, an independent from Vermont who caucuses with Democrats, has been testing the waters for a 2016 bid. He's hoping to push an agenda that focuses on income inequality, climate change and campaign finance reform. Sanders has already traveled to early primary states such as Iowa and New Hampshire. Win McNamee/Getty Images
Former Virginia Sen. Jim Webb has said he's "seriously looking" into a 2016 bid for the Democratic nomination. Webb left the U.S. Senate at the end of his first term in January 2013. He's a veteran of the Vietnam War who also served as Navy secretary and assistant defense secretary. TOSHIFUMI KITAMURA/AFP/Getty Images