Democratic presidential candidates, Hillary Clinton, left, former Maryland governor Martin O'Malley, center, and Sen. Bernie Sanders, I-Vt.), pose for a photo at an earlier debate. (AP Photo/Chuck Burton)

MANCHESTER, N.H. -- Hillary Clinton was the only one of the three Democrats on stage Saturday night willing to pledge that she wouldn’t raise taxes on those making less than $250,000 a year.

“I don’t think we should be imposing new big programs that are going to be raising middle-class families’ taxes,” Clinton said during the Democratic debate, saying she would stick would stand by a similar promise made by President Obama.

Her chief rival for the Democratic nomination, Sen. Bernie Sanders of Vermont, defended his support of a plan for to guarantee paid family leave that would result in a modest payroll tax increase for all workers.

Sanders also wants to move to a “Medicare for all” health-care system under which taxes would increase for many middle-class Americans. But Sanders argued the overall cost of care would go down for most people by “thousands of dollars” because they would no longer pay premiums or co-pays.

[Clinton ramps up efforts to undercut Sanders with middle-class voters]

Clinton has sought to make her stance a defining issue in the race for the Democratic nomination against Sanders and former Maryland governor Martin O’Malley.

During the debate, Sanders pushed back on family leave, arguing that the “vast majority of progressive Democrats” in both chambers of Congress support a plan introduced by Kirsten Gillibrand, Clinton’s successor as a senator from New York.

The bill, which would guarantee three months of leave after a family has a child, would be paid for by an increase in the payroll tax estimated to cost the average worker about $72 a year, according to Gillibrand.

Sanders said the average worker would pay $1.61 a week more.

“You can say that is a tax on the middle class,” Sanders said, adding: “I think a buck-sixty-one is a pretty good investment.”

Clinton said she has been a long-time proponent of family leave but has found a way for the wealthy to pay for it.

O’Malley said he would not make a blanket guarantee about taxes.

“No, I’ve never made a promise like that,” the former governor said.

He noted that in Maryland, he had pushed through a more progressive income tax plan.

Left unmentioned were several tax increases he backed that also hit those making less than $250,000 a year, including increases in the sales tax, gas tax and alcohol tax, as well as toll increases on Maryland roads.