Elizabeth Warren is rising to the top of the Democratic pack with ambitious promises to reshape the political and economic system. But as she faces growing scrutiny, the Massachusetts senator is opening herself to criticism that she's just another politician dodging the tough questions. She is in a bind because of her persistent refusal during two straight presidential debates to say whether she would raise taxes on the middle class to pay for the universal health insurance plan known as 'Medicare for All.' By not acknowledging taxes would almost certainly increase for a wide range of income earners, Warren avoids becoming a caricature of a Democrat itching to raise them. But she also threatens to undermine the image she's fostered of a plainspoken former professor ready to tackle any issue in her quest to protect the middle class from the excesses of corporations and the wealthy. Warren's progressive rival, Bernie Sanders, has said middle-class taxes would have to rise to pay for Medicare for All. Other White House hopefuls said Wednesday that Warren should be just as direct. 'Look, I'm not picking on Elizabeth Warren, but this is ridiculous,' said former Vice President Joe Biden, currently Warren's chief competitor for the Democratic nomination. Warren 'is going to have to tell the truth' or face questions about her willingness 'to be candid and honest with the American people.' Minnesota Sen. Amy Klobuchar said Warren 'needs to come forward and say' how she'd pay for a new health insurance system. 'I'm sure she will eventually,' Klobuchar said. Warren argues pundits are missing the point by focusing on taxes instead of the bottom-line cost that Americans pay for their health care. She insists that eliminating premiums and co-pays under Medicare for All would lower overall costs for all but wealthy Americans. Her top supporters say she should keep pressing that message. 'Democratic voters are actually very appreciative that, on the substance, she wants to bring down health care costs, and, on the politics, she's not taking the bait and giving Republicans and the insurance industry the TV ad moment that they want to deceive voters,' said Adam Green, a liberal activist and close Warren ally. Still, the lack of specificity on paying for Medicare for All is tricky since Warren famously is the candidate who 'has a plan' for everything and proudly sweats even the smallest, wonkiest details. On health care, she says that she's 'with Bernie,' referring to the Vermont senator who authored the Medicare for All legislation in Congress. Warren's campaign says that no one yet knows Medicare for All's final price tag — but that Warren is still 'reviewing the revenue options' previously suggested by Sanders and that she has been very consistent and clear in saying she'll pay for it by adhering to the principles of lowering overall costs for middle-class families and raising them for rich people and major corporations. Democratic presidential candidates traditionally go to great lengths to avoid the idea that their party is tax happy. But being blunt about raising taxes is not necessarily an election loser, as long as their effect is targeted. President Barack Obama was reelected in 2012 following a pledge to raise taxes on top earners and did so by allowing previous tax cuts on the wealthy to expire. And Warren's promises of a 2% wealth tax on households with a net worth of more than $50 million elicits chants of '2 cents!' at her rallies, bolstering her reputation as an economic populist. The generous benefits Warren is promising — surpassing other countries with government-run health care — would require tax increases of a historic magnitude to guarantee cradle-to-grave care for every U.S. resident, however. And that will make it harder to finance the program with surgically targeted tax increases on corporations and the wealthy. A study released Wednesday from the Commonwealth Fund and the Urban Institute estimated the government would need $2.7 trillion in additional revenues if Medicare for All were fully implemented next year. That's more than half the current federal budget, and Washington is already borrowing heavily to meet its obligations. 'It's a huge tax increase,' said Urban Institute health economist John Holahan. 'If you just lay it on a small group of people, how do you get those numbers?' Targeting income brackets also means necessarily picking winners and losers — and a person's health status at any given time could determine what side of the ledger they wind up on. 'The success or failure of major health reform plans always hinges on who would win and who would lose, and we don't have the detail for the Medicare for All plans to judge that,' said Larry Levitt, a senior health policy expert with the Kaiser Family Foundation. Support for Medicare for All has fallen slightly since the beginning of the year, even as Warren and Sanders have championed it. An October poll by the Kaiser Foundation found 51% of Americans favor the plan, while 73% said they support a public option that would compete with private health insurance. Among Democrats, support for a public option has outpaced support for Medicare for All, 85% to 71%. A public option is backed by more moderate Democratic hopefuls, including Biden and Pete Buttigieg, mayor of South Bend, Indiana. Traditionally, the U.S. has paid for social insurance programs with broad-based taxes. Medicare's financing includes a payroll tax, money from the Treasury's general fund and premiums paid by seniors. Sanders has laid out some options for Medicare for All, including a payroll tax on employers, a 'premium' tax on households making more than $29,000 for a family of four and higher taxes on the wealthy — which is further than Warren has gone in explaining where the funding will come from. Releasing a more detailed health care proposal of her own could at least start to address these issues for Warren. But her advisers have long maintained that, while she may eventually do so, it isn't imperative. 'What resonates is not an academic version of her plan — it's her personal story of struggle connecting with the real-world impact of her plan on everyday families,' Green said. 'Medicare for All is about the principles, not the micro-mechanisms for putting it altogether.' ___ Associated Press Writers Emily Swanson in Washington, Hunter Woodall in Keene New Hampshire and Julie Carr Smyth in Columbus, Ohio contributed to this report.