Barack Obama and John Edwards try to paint her as a candidate of the Washington establishment and beholden to special interests. Chris Dodd questioned the former first lady's competence on health care reform. They have hinted she's too divisive to govern effectively as president.
The shift in tone was perhaps inevitable, coming nine months into a largely cordial primary campaign that has left the New York senator the undisputed front-runner for the Democratic nomination. In criticizing Clinton, they acknowledge she's a formidable candidate and accept that she's unlikely to stumble badly to give others an opening.
"Her opponents are starting to worry that she is consolidating her position, and that's potentially fatal for them," said Raphael Sonenshein, a political science professor at California State University, Fullerton. "A lot of people watching her campaign are surprised by the fact that it's strengthening and could be starting to break away."
The new dynamic is also a clear expression of frustration by Clinton's rivals, who were forced to the sidelines this week when she released her new health care plan. The rollout drew extensive media coverage.
Republicans are criticizing Clinton as though she's already the Democratic nominee. Rudy Giuliani has relied on newspaper and Web ads to assail her on the Iraq war.
To remain in the game, Democrats are starting to point out Clinton's potential vulnerabilities and question her electability.
At a seniors' forum in Iowa on Thursday, rival Joe Biden suggested congressional Republicans would refuse to work with Clinton to accomplish health care reform.
"Let's be frank about this," Biden said. "What's changed to make you think Hillary is going to be able to put together the 15 percent of Republicans" who will be needed to enact any overhaul of the health care system?
In an interview with The Associated Press, Dodd said Clinton had mismanaged her effort to reshape the nation's health care system during her husband's presidency and questioned why she touted that experience as evidence she should be allowed to try again.
Biden and Dodd are both polling in single digits and have had little impact on the overall dynamic of the field. But their criticisms come amid new efforts by Clinton's lead rivals, Obama and Edwards, to portray her as part of the status quo.
Edwards has been particularly aggressive, claiming Clinton lifted his health care plan and criticizing her ties to lobbyists and other special interests.
His top campaign strategist, Joe Trippi, even sent an e-mail to supporters this week blasting her for attending a fundraising lunch with lobbyists. Clinton, he wrote, is the "poster child" for what's wrong in Washington.
Obama faces his own set of risks and complications. He has pledged to run a positive campaign without the personal attacks or negativity that would cast him as a "conventional" politician. That pledge has come with a downside: Clinton strategists pounce each time Obama utters any sort of critique.
So in a new television ad campaign released this week, Obama tiptoes around Clinton's vulnerabilities without addressing them head on.
In an ad about health care, he laments the "bickering" that defined past attempts to reform the system. "For the last 20 years, Washington has talked about health care reform but reformed nothing," he says.
Is he talking about Clinton? Obama doesn't say.
And without naming names, Obama's new campaign speech includes a warning about a return to political polarization.
"George Bush and Dick Cheney may have turned divisive, special interest politics into an art form, but it was there before they got to Washington," he said.
For her part, Clinton hasn't taken the bait and has largely ignored the potshots from fellow Democrats.
"Voters can see through politically motivated attacks," said her spokesman Howard Wolfson. "Other candidates are clearly frustrated with their falling poll numbers."
Clinton is already training her sights on the GOP. She referred to Cheney this week as Darth Vader, and a top Clinton campaign adviser, former Iowa Gov. Tom Vilsack, said Giuliani's rocky personal life would be fair game in a general election.
But longtime Democratic strategist Erik Smith said that despite Clinton's clear strengths, there was still opportunity for her rivals to make headway. Their critiques could have particular resonance in the early voting states of Iowa and New Hampshire, where voters are closely following the contest, he said.
"If you are running behind a front-runner, you have to do something to change the dynamics of the race," Smith said. "You really have no choice - you can't rest on your laurels and hopes that she trips."