Who is blocking a grand debt deal? Democrats, too, have their limits.

Republicans who refuse to consider tax increases aren’t the only obstacles to President Obama’s bid to get a significant debt deal through Congress in time to avoid default as early as Aug. 2.

Democrats, too, have a point beyond which they will not go – cuts to Social Security and Medicare benefits – and want the president to keep it in mind as he prepares for a new round of negotiations at the White House on Sunday.

The White House has asked that congressional leaders lay out their bottom line negotiating positions at the Sunday meeting.

For that reason, a closed caucus meeting on Friday was a last chance for many Democrats to signal the White House and their own leaders what it will take to win their votes, once a deal comes to the floor.

“I came to Washington to protect Social Security and Medicare, not to dismantle them,” says Rep. Jim McGovern (D) of Massachusetts. “These aren’t just programs, they represent core values of the Democratic Party. You shouldn’t be balancing the budget on the backs of seniors and poor people.”

Democrats say they don’t know what’s been discussed in closed meetings so far, but are alarmed by what they’re hearing in the corridors or reading in press reports. Republicans, too, say they are in the dark. The White House and congressional leaders have deliberately avoided discussing details before the deal is done.

But the rank and file in both parties want to make it very clear to their leaders what will fly on the floor, when it’s necessary to find a majority of 218 House votes. Democrats say they expect that Republican Speaker Boehner will need at least 100 votes from Democrats to pass a debt deal. That, they say, gives them some clout.

“The know-nothing wing of the Republican Party is going to vote against a debt ceiling agreement no matter what is in it,” says Rep. Gerald Connolly (D) of Virginia, alluding to freshmen Republicans who campaigned on a pledge to never raise the debt ceiling. “Speaker Boehner is going to need the Democratic caucus to pass this. That means that, whether he likes it or not, taxes are on the table.”

From the start of the debt talks, Boehner has said that tax increases are off the table, because it hurts jobs to raise taxes when the economy is struggling to recover from a recession. At a press briefing on Friday, he said that his aim from the start of the process was “the big deal that would fundamentally solve our spending problem and our debt problem in the near to medium term.”

“But at the end of the day, we’ve got to have a bill that we can pass through the House and the Senate,” he added. “In all honesty, I don’t think this problem has narrowed at all in the last several days.”

Back in the days when she was House speaker, Nancy Pelosi (D) of California was the president’s essential partner on Capitol Hill, but as the House minority leader recently has been less a presence in closed White House negotiating sessions, displaced by Boehner.

Now, she’s a player again, if only because Democratic votes may be needed when it comes time to pass a debt deal in the House.

Democrats want to reduce the budget deficit and grow the economy, but not “on the backs of America’s seniors and working families,” Pelosi said Friday. That means “no benefit cuts in Medicare and Social Security,” she said. “And we have serious concerns about what is happening with Medicaid as well.”

“I'm still optimistic that we can find a place where we can come together, I don't like to have a situation where we're saying, well, you need our votes, so you better have this in the bill,” she added.

House Republicans have taken a pounding in public opinion polls over their budget plan for FY 2012, which reduced the federal role in Medicare. They lost a special election in New York they had been expected to win that turned on GOP Medicare cuts.

Democrats are gearing up to use the issue in the 2012 campaign – a strategy that would be undermined if Democrats wind up voting for a debt deal that includes Medicare cuts.

“A lot of Democrats thought they were going to run [in 2012] as guardians of Medicare and Social Security, but for the White House to tell them that both are going to be chopped puts their reelection plans in jeopardy,” says Thomas Ferguson, a political scientist at the University of Massachusetts, Boston.