Hillary Rodham Clinton’s backers and allies are concerned that each of the emerging challengers for the 2016 Democratic nomination appeal to a constituency within the party that she has struggled with in the past. (Bebeto Matthews/AP)

Backers and allies of Hillary Rodham Clinton are increasingly worried about the threat posed by a motley field of Democratic presidential hopefuls who could complicate, or even derail, a Clinton candidacy in 2016 by focusing attention on her weaknesses.

All of the possible challengers are long shots against Clinton and would face a steep climb against the well-known former secretary of state. Many Clinton supporters also say competition would help her by honing her campaigning skills and discouraging the sense of entitlement that damaged her White House bid in 2008.

But each of the emerging challengers also appeals to a constituency within the Democratic Party that Clinton has struggled with in the past. And unlike Clinton — who has yet to formulate a clear message for a potential campaign — each has distinct issues to build a campaign around.

Jim Webb, the former senator from Virginia who just formed an exploratory committee, is a populist native of Appalachia with potential appeal to working-class and Southern whites. Maryland Gov. Martin O’Malley has been laying the groundwork of a campaign for months, focusing his energies on wooing the kind of progressive activists who view Clinton with suspicion. Sen. Bernard Sanders (I-Vt.), the gadfly socialist who is also pondering a run in the Democratic primaries, represents the antiwar left still bitter with Clinton over the war in Iraq.

Longtime Clinton family political adviser Harold Ickes said it would be a mistake to dismiss such challengers and the dangers they pose.

Jim Webb, the former senator from Virginia, has the potential to appeal to the working-class and Southern whites. (Chip Somodevilla/Getty Images)

“[What if] this were 2007 before Obama got into the race and you’d said, ‘Do you think Senator Obama is a threat to Hillary?’ ” Ickes asked rhetorically. The clear answer, he suggested, is that most would have dismissed Obama as little more than an annoyance.

But the biggest concern among many Clinton acolytes is someone who says she is not running — Sen. Elizabeth Warren (Mass.), an economic populist who has come to personify a longing among liberal Democrats for someone further to Clinton’s left.

Warren especially interests and worries Bill Clinton, the unofficial top strategist for his wife’s shadow campaign, according to two people who know the former president well. Bill Clinton admires Warren’s stemwinder speaking style, and Hillary Clinton echoed parts of Warren’s sticking-up-for-the-little guy economic message during midterm speeches this year.

During their one midterm appearance together, Clinton lavished praise on Warren and kept her own remarks brief. Elsewhere, she tried out appeals to working-class and underemployed voters that strategists expect to hear again if Clinton runs.

Many Clinton backers insist that some Democratic opposition is both inevitable and welcome, since it tends to toughen up the eventual winner for the head-to-head contest with a Republican in the general election. Looking at the lessons of Clinton’s bitter primary contest with Obama in 2008, Democrats also hope that Clinton will be polite, even deferential, to potential opponents such as Webb if she runs.

Loyalists to Obama and Clinton privately agree that Obama snatched the nomination away from Clinton in part because of her campaign’s failure to see the upstart as a real threat.

“I have never assumed, and I think anybody would have been in error to assume, that our party would just give its nomination to anyone,” said Craig Smith, a longtime adviser to both Clintons who is now working for the Ready for Hillary super PAC.

Sen. Elizabeth Warren (D-Mass.) says she won’t run, but the economic populist has come to personify a longing among liberal Democrats for a candidate that leans further to the left. (Timothy D. Easley/AP)

“That is not the history of our party. This isn’t how it works,” Smith told reporters in New York at a November gathering of potential donors and workers for a Clinton campaign. “You’ve got to go out there. You’ve got to work for it.”

Speaker after speaker at the Ready for Hillary-sponsored event tried to dispel the notion that Clinton will walk away with the nomination. Some strategists outside the group, however, note that its very existence reinforces the notion that she is the de facto choice. No other potential candidate has anything like the pro-Clinton machinery, which includes three major outside groups that have amassed money and an impressive list of potential supporters.

Webb — Navy secretary in the Reagan administration, a combat veteran, an author and a filmmaker — became the first Democrat to formally jump into the race with the release last month of a lengthy Internet video and the formation of an exploratory committee.

Webb did not mention Clinton in his video but appeared to take a few shots at her as the establishment favorite. Government is “paralyzed,” Webb said, and he wants to shake it up. He made a point of saying he is a public servant, not a “career politician.

“In my view, the solutions are not simply political but those of leadership,” Webb said. “I learned long ago on the battlefields of Vietnam that in a crisis, there is no substitute for clear-eyed leadership.”

Like Clinton, Webb is considered a strong defender of American military power who is moderate on social and economic issues. They occupied similar political space in the Senate, but Webb, who has been critical of recent U.S. wars, may have special appeal to white working-class and Southern voters whose interests he has long championed. Webb’s exploratory committee did not respond to requests for comment.

O’Malley, who leaves office in January, touts himself as a can-do executive who oversaw a wave of progressive policies in Maryland, including legalizing same-sex marriage, abolishing the death penalty and raising the minimum wage.

Many analysts believe O’Malley would be likely to position himself as the second candidate, ready if Clinton stumbles. He has attended more than two dozen political events this year in the early caucus state of Iowa, where Clinton was trounced in 2008.

Sanders’s political niche would be as an antiwar conscience candidate, highlighting Clinton’s 2003 vote to approve the Iraq war, her support of the war in Afghanistan and her role in helping guide the campaign in Libya while secretary of state. U.S. military involvement in Syria came after Clinton had stepped down, but she has said publicly that she supported an intervention much earlier.

One danger, several strategists said, is that Clinton might be lured into espousing base-friendly positions that would hurt her in a general election, as GOP nominee Mitt Romney did in advocating strict immigration measures during the 2012 Republican debates. Perhaps a bigger risk, they said, is that a savvy primary opponent with a sharply honed message or a fresh face could upstage her.

By comparison with those of Republicans, whose tea party wing tends to pull centrists to the right, the differences among Democrats are more on the margins, many Clinton supporters said. That will make it all the more important for her to take her opponents seriously — at least on the surface — and use them to show her strengths as a capable leader who is up to the challenge of pulling out a third Democratic general-election win in a row, according to the strategists.

Clinton is watching the potential field — both Democratic challengers and possible Republican opponents — but they are essentially irrelevant to her decision about running, friends said. They also said Clinton is genuinely undecided and is still mulling the decision of whether to run.

Focus on Clinton’s plans, already intense, will increase after Christmas. She has said she is likely to make a decision after Jan. 1. That’s when other potential challengers to Clinton could emerge. Although the 2016 election is still far off, other hopefuls have less leeway to wait.

“I think there are people in the party who would like to know sooner rather than later” what Clinton is going to do, Ickes said, starting with Webb and O’Malley.

“Unlike Hillary, they don’t have . . . a national political apparatus, nor do they have a national money base,” Ickes said. “And you know, there are money people who are going to say, ‘I’m not giving to anybody until I know what Mrs. Clinton is going to do.’ ”

Although Clinton backers snicker a bit about Webb — one strategist called his Internet announcement a “14-minute hostage video” — they do not discount his appeal as a public servant with an independent streak. That same strategist also said Webb gains stature simply by virtue of challenging Clinton. “If it’s one-on-one, he will, by default, be a serious candidate,” said the strategist, who requested anonymity because Clinton has not yet said she is running.

“There will be an opposition, and that opposition needs some outlet,” he said. “He can be the vessel of that opposition, and then when Iowa falls apart for him, for example, that’s it.”