Biden tops list of potential 2020 Democrats in new poll
Five figures on the left who could challenge Hillary Clinton
Hillary Clinton's criticism of President Obama this week has added to the skepticism about her that has long percolated through the Democratic Party's grassroots.
Clinton's assertions of the need for a muscular foreign policy in her interview with The Atlantic also caused a stir among liberal-leaning commentators.
Ezra Klein wrote that her key weakness as a candidate is that "she is more hawkish than the post-Iraq Democratic Party...She sounds like a Democrat from 2002 rather than 2014."
Clinton remains a dominant frontrunner in the race to win the party's presidential nomination in 2016, if she runs. A CNN poll released last month indicated she had the support of fully two-thirds of the party's supporters while other potential contenders struggled to break out of single digits.
Clinton, of course, was also a runaway favorite for the 2008 presidential nomination until Obama's candidacy caught fire and carried him all the way to the White House.
The key question for liberals now is who might conceivably emerge to give Clinton a serious challenge from the left. Here are five possibilities.
Sen. Elizabeth Warren (D-Mass.)
Warren has long been a hero to the liberal grassroots, a status that was further cemented by her appearance at the Netroots Nation convention earlier this summer.
Warren delivered a fiery speech on 11 principles of modern liberalism that delighted a crowd that chanted "Run, Liz, run."
Among those principles was her belief "that Wall Street needs stronger rules and tougher enforcement - and we're willing to fight for it" - the political cause with which she is most closely identified.
The problem for progressives is that Warren has given no real indication that she will run in 2016 and plenty of evidence that she will not. In April, she told CBS News that change was needed in the United States "right now" but then added, "You can ask this in a whole lot of different ways but the key is - I'm not running for president."
Vice President Biden
Biden is in an unusual position for any vice-president: He has signaled that he wants the top job but he is widely regarded as unlikely to get it.
His boosters point out that he has a number of achievements that should appeal to the grassroots. The most conspicuous in recent years was his public announcement of support for same-sex marriage in May 2012. This was credited in liberal circles with nudging Obama toward a similar public profession of support, which he made just days later.
At the same Netroots Nation that was wowed by Warren, Biden also asserted, "I don't take a backseat to anyone when it omes to fighting some of the toughest progressive battles the country's seen."
But Biden would have to overcome a lot of challenges in a 2016 run, prime among them a sense that his career is nearing its end rather than its apex. He first ran for president in 1988 and, when he tried again 20 years later, he exited after placing fifth in the Iowa caucuses.
Running against Hillary - to whom he is personally close - would open Biden up to a potential humiliation. He might well prefer not to risk it.
Maryland Gov. Martin O'Malley (D)
O'Malley is not the kind of fire-breathing populist who excites the left but he has also been more overt than some other contenders when it comes to his 2016 ambitions.
While most possibly candidates have offered only veiled hints about their intentions, O'Malley told reporters a year ago that he was laying the "framework" for a presidential run.
"He's made it clear to me he's running. We did not discuss what he would do if Hillary Clinton runs," a supporter told The Hill at that time.
Last month, O'Malley positioned himself to the left of the current administration over the immigration crisis on the southern border. "It's contrary to everything we stand for to try to summarily send children back to death," he asserted.
Whether O'Malley could really marshall the firepower to mount a serious challenge to Clinton is doubtful, however.
Sen. Bernie Sanders (I-Vt.)
Would Sanders enter the race thinking he had a legitimate shot at ending up in the White House? Probably not.
But he could run as a tribune of the left, representing what both former Vermont Gov. Howard Dean and the late Sen. Paul Wellstone (D-Minn.) liked to call "the Democratic wing of the Democratic Party."
Sanders could make life distinctly uncomfortable for Clinton, even if he had little chance of prevailing against her.
Earlier this month, he delivered a clear shot across her bows, telling Yahoo and ABC News, "She has accomplished a lot of very positive things in her career but I'm not quite sure that the political process is one in which we anoint people."
Former Sen. Russ Feingold (D-Wis.)
Feingold has not been on anyone's radar of late. A liberal icon at the time he piloted campaign finance reform through the Senate with Sen. John McCain (R-Ariz.), Feingold has been serving in a little-noticed State Department role - he is special envoy to the Great Lakes region of Africa.
Feingold would have an infrastructure, of a kind, if he decided to run: he founded the Progressives United PAC in 2011, its stated aim being "to combat the corrupting influence of corporate money in our elections and government."
That kind of rhetoric has only become more popular on the left of the Democratic Party and Feingold could not be easily dismissed as a gadfly candidate.
As with Sanders, his chances of winning would be slim but his capacity to discomfit Team Clinton would be considerable.