WESTERVILLE, OHIO — Polls lately have begun to suggest that Sen. Elizabeth Warren (D-Mass.) might have found a glide path to the Democratic presidential nomination. But on Tuesday night, she hit some turbulence.

What’s troublesome for Warren is not the fact that she found herself the chief target of attacks from her rivals in the fourth Democratic debate; that was, in a sort of backhanded way, a tribute to her newfound status as a leader — perhaps the leader — in a crowded field of candidates.

More ominous was her repeated evasion of a question put to her by moderator Marc Lacey of the New York Times: Would her Medicare-for-all proposal require an increase in taxes on the middle class?

Instead of answering what was a straightforward, yes-or-no question, Warren talked about her “principles,” which include lowering costs for average families and raising them for corporations and the wealthy.

Instead, she talked about the people she has met as she has campaigned in 27 states and Puerto Rico, doing 140 town halls and posing for some 70,000 selfies, “which must be the new measure of democracy.”

“The way I see this, it is about what kinds of costs middle-class families are going to face,” Warren said. “So let me be clear on this. Costs will go up for the wealthy. They will go up for big corporations. And for middle-class families, they will go down. I will not sign a bill into law that does not lower costs for middle-class families.”

It was pretty much the line she has been getting away with offering for months, starting at a time when she was running well behind both former vice president Joe Biden and Sen. Bernie Sanders (I-Vt.).

Her calculation was easy to understand. What Warren is trying to avoid was producing a sound bite that will play in thousands of Republican attack ads. And owning up to proposing a middle-class tax increase would undercut her rationale for running, which is as a populist who would reset a system in which everything favors the rich and the powerful.

But a front-runner, especially one who styles herself as a policy wonk, gets no passes.

Mayor Pete Buttigieg of South Bend, Ind., was the first to leap. “This is why people here in the Midwest are so frustrated with Washington in general and Capitol Hill in particular,” he told her. “Your signature, Senator, is to have a plan for everything. Except this.”

Biden accused her of being “vague,” and Sen. Amy Klobuchar (D-Minn.) added: “We owe it to the American people to tell them where we’re going to send the invoice.”

But the most devastating blow was delivered by Sanders. At the first debate in June, when he was still ahead of her in the polls, Warren had summed up her own stance on health care by saying, “I’m with Bernie on Medicare-for-all.”

“As somebody who wrote the damn bill, let’s be clear: Under the Medicare-for-all bill that I wrote, premiums are gone, copayments are gone, deductibles are gone. All out-of-pocket expenses are gone,” Sanders said Tuesday. Then he added: “But I do think it is appropriate to acknowledge that taxes will go up,” including for middle-class Americans.

Among the options that Sanders has listed for financing his plan is a 4 percent “income-based premium” to be paid by households making more than $29,000 a year. He insists that the cost would be far lower than what they are paying in health insurance premiums, which would be eliminated under a government-financed system.

Warren cannot put this question behind her by evading it. All she has done is guarantee that it will come up over and over again, and the longer she dodges, the more she will raise larger doubts about whether she is as forthright as she has portrayed herself to be.

At a critical moment when Americans have begun to take her measure as a real possibility to become the next president of the United States, Warren’s job is to show not only that she has done her homework but also that she is someone they can trust. She should have a plan for that.

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