WASHINGTON (AP) — A coy Elizabeth Warren is withholding an endorsement of Hillary Clinton or Bernie Sanders for now and using her leverage as liberal icon to focus the two Democratic presidential rivals on her populist agenda.

On the eve of Tuesday's New Hampshire primary, both contenders would love to win the backing of the Democratic senator from next-door Massachusetts. Warren's clout flows from the army of progressive voters who'd hoped she'd seek the White House herself, plus her longtime outrage over what she considers a rigged financial system, which taps directly into one of this year's dominant Democratic themes.

"There are a lot of people who will be listening very carefully to how she feels about the race," said Sen. Brian Schatz, D-Hawaii, a Clinton backer.

Yet many Democrats say Warren faces a difficult decision best addressed by letting the race percolate while using her influence to inject her concerns about income inequality, financial regulation and student debt into the debate.

While many of Sanders' issues mirror Warren's views, backing the Vermont senator would align her against Clinton, seen by most Democrats as their likely nominee. A President Clinton would be crucial to advancing Warren's policy goals.

Warren and Clinton have had their disagreements, but each advocates a robust government that tackles social and economic problems. Yet endorsing Clinton while Sanders remains a viable candidate could disillusion Warren's exuberant young and liberal supporters, many of whom now are embracing Sanders.

"Elizabeth will make her decision on her own time," said Sen. Amy Klobuchar, D-Minn.

Warren is the only one of the Senate's 14 Democratic women not backing Clinton, the former secretary of state, New York senator and first lady.

Privately, many Democrats said they wonder whether Warren regrets not running herself when she sees how competitive Sanders, her ideological doppelganger, has been. As a candidate, she could have fed off the same anti-establishment sentiment fueling Sanders' bid while adding the broader appeal of not being a self-proclaimed socialist and being a woman without the questions about emails that have nagged Clinton.

Warren declined to speak to a reporter for this story, and aides did not agree to requests to respond on the record to questions.

The Oklahoma City daughter of a janitor, Warren, 66, became a champion of consumer financial issues, author and Harvard law school professor. She pioneered the idea that became the Consumer Financial Protection Bureau, which was created by President Barack Obama's 2010 law overhauling federal financial regulations, and spoke at her party's 2012 presidential convention.

Warren arrived in the Senate in 2013 after defeating Republican Scott Brown for the seat previously held by the late Edward Kennedy, inheriting Kennedy's mantle as a hero of the left.

Clashes with Obama over his Pacific Rim trade treaty and other issues have only strengthened her ties to liberals, and she's become a potent fundraiser for Democrats and progressive causes. Now, she's using her influence to push her concerns to the 2016 forefront.

Two weeks ago, Warren released a report alleging lax federal treatment of corporate law-breakers and wrote an opinion column calling the candidates' attention to the subject in The New York Times.

"The next president can rebuild our faith in our institutions by honoring the simple notion that nobody is above the law, but it will happen only if voters demand it," she wrote.

Most Democrats expect Warren to endorse Clinton but concede they are not certain. They say the relationship between the two women is wary, fed by a mix of clashes and compliments.

Warren joined other Democratic senators in a 2013 letter to Clinton urging her to run for president. But she was the only female Democratic senator who did not attend an event last November where the others endorsed Clinton.

Clinton has received huge campaign donations and speaking fees from the same financial giants Warren often combats. And Warren has said Clinton, while senator, reversed earlier opposition to Wall Street-backed bankruptcy legislation, a turnabout Clinton has attributed to improvements in the bill.

Yet Warren has praised Clinton for proposing expanded Wall Street regulations and new levies on U.S. companies that merge with foreign firms to reduce their tax bills.

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This story has been corrected to remove incorrect reference to Warren's mother working as waitress.