If Democrats in deficit negotiations agree to a compromise that cuts Medicare benefits to seniors, they risk squandering the advantage they’ve built up over Republicans on the issue since 2010 and risk losing their more general edge as defenders of the middle class, a top Dem pollster who just completed an extensive health care poll tells me.

Jeff Liszt, of the respected Dem firm Anzalone Liszt, has just completed a poll for two liberal-leaning groups finding that the Paul Ryan Medicare plan is deeply unpopular with voters, and particularly with seniors and independents, when the plan is described to them. The poll also found that Obama and Dems have increased their advantage over Republicans on Medicare, on health care in general, and on who can be trusted to defend the middle class.

I followed up with Liszt to ask whether his polling indicated that Dems could lose those advantages if they agree to a deficit reduction deal that cuts Medicare benefits and shifts costs to seniors. His answer was unequivocal.

“Agreeing to benefits cuts takes the foot off the gas in terms of going on the offensive against Republicans,” said Liszt, who did the poll for the Herndon Alliance and Know Your Care. “You have to draw a bright line somewhere and Medicare benefits are the best place to do that.”

Liszt pointed out that Republicans had made big gains in 2010 by accusing Dems of cutting Medicare in the Affordable Care Act, and said his new poll showed that Ryancare had enabled Dems to successfully rebuild trust with voters since then on health care and as defenders of the middle class. Liszt said that Republicans would continue to attack Dems from the left on Medicare no matter what they agree to in deficit talks, but warned that agreeing to actual Medicare benefits cuts could make it easier for Republicans to reverse their losses on the issue.

“Republicans are going to make the same argument irrespective of whether or not Democrats agree to more cuts,” he said. “But I still think [agreeing to benefits cuts] would be politicaly problematic for Democrats right now. They’re in a period where they’re building their advantage on health care issues. It’s an important component of the Democratic advantage on fighting for the middle class, which was central to Democratic victories in 2006 and 2008 and eroded in 2010.”

Said Liszt: “Benefits cuts could set that back.”