She has been described as "Hillary's nightmare" - a banker-bashing, consumer-championing United States senator who liberals believe could be a serious challenge to Clinton in the 2016 presidential election.
Officially, Elizabeth Warren, a 64-year-old former Harvard law professor and Massachusetts senator, has no White House ambitions.
"I'm not running for president," she told ABC News on Monday.
But she was in the TV studios to promote a new autobiography that, in the tradition of books such as Barack Obama's The Audacity of Hope, sounded to many pundits like a campaign manifesto. And in Washington circles, her denials that she would run succeeded only in deepening speculation that she might.
The publication of A Fighting Chance, which tells the story of Warren's hardscrabble upbringing in Oklahoma, comes six weeks before Clinton's campaign book, Hard Choices, hits bookstores in June.
Asked if she thought the former First Lady would make a good president, Warren stopped short of unequivocal endorsement, replying: "I think Hillary Clinton is terrific."
Warren, a figurehead of the liberal left and popular on late-night TV chat shows for her attacks on Wall Street and income inequality, has been spoken of as a possible presidential contender ever since her barn-storming speech at the Democrat convention in 2012.
She has also attacked an economy that she says is rigged in favour of big business, a message that polls show is popular among the disgruntled middle classes who have seen little recovery in their own wages since the end of the 2007 financial crisis.
"Big corporations hire armies of lobbyists to get billion-dollar loopholes into the tax system and persuade their friends in Congress to support laws that keep the playing field tilted in their favour," Warren writes. "Meanwhile, hard-working families are told they'll just have to live with smaller dreams for their children."
Although Warren was among the 16 female Democrat senators who signed a secret letter last year urging Clinton to run for president, her deeply populist message contrasts with the Clintons' establishment profile and Wall Street ties.
She has made much of her own tough upbringing as a carpet salesman's daughter who attended government schools, was a waitress at 13, got married at 19 and lived on the "ragged edge" of the American middle classes.
Warren cuts a solo figure in public life, and her family made few appearances on the campaign trail when she ran for the Senate in 2012.
She has spoken of the difficulties of being a working mother of her two grown-up children from her first marriage to computer engineer Jim Warren, who she started dating in secondary school.
The marriage ended in 1978, but she kept the Warren name when she married Bruce Mann, a law professor at Harvard, in 1980.
Her book has been described by reviewers as a "campaign call to arms" and a "manifesto not-so-subtly announcing a politician's aspirations for higher office".
But Warren has dismissed talk of a presidential bid as the idle chatter of "pundit world" and urged greater focus on core economic issues facing a nation where 95 per cent of income gains between 2009 and 2012 went to the wealthiest 1 per cent.
But still the pundits did chatter, arguing whether Warren - a brilliant fundraiser - might be considering a bid, or was merely keeping her options open in case Clinton decided not to run herself.
Curiously, Noam Scheiber, the conservative New Republic magazine pundit who coined the phrase "Hillary's nightmare", said that after reading Warren's book and noting the favourable way she portrayed Clinton, he had now concluded Warren would not run after all.
Whatever her decision, Clinton remains the front-runner. A Fox News poll last week found 69 per cent of Democrats backed her for the nomination, ahead of Vice-President Joe Biden on 14 per cent and Warren on 6 per cent.
They are numbers that one pro-Clinton strategist waspishly suggested might explain Warren's real strategy: "She's trying to sell some books and keep herself relevant, is my guess. She would be mad to run against Hillary, and she knows it."