House liberals are still holding out hope that health care reform can include a public insurance option, even though the issue isn’t even on the agenda at President Barack Obama’s high-profile health care summit Thursday.

“I’m pleased the president is having an open dialogue. But the fact is, at the end of that dialogue, we will have to come to some form of agreement. If Republicans don’t support it, we may well go back to moving forward and solving the problem with or without Republicans,” Rep. Chellie Pingree (D-Maine) said in a conference call.

Pingree said momentum is building for the public option and it is still possible that Republicans could get on board with the idea, although none has thus far. She pointed to Wednesday’s overwhelming vote in the House to scrap the insurance industry’s 65-year antitrust exemption. The 406-19 vote split GOP leadership and gave Democrats new footing on their larger health care reform push.

“Sometimes you have to advance an idea and show how much support there is in order to advance it,” Pingree said.

The public option has picked up some steam in recent weeks: More than 120 House Members and 24 Senators — some of whom are participating in Thursday’s summit — have signed on to the idea of using reconciliation to attach a public option to Obama’s health care plan.

Pingree, a member of the Rules Committee, said there are a number of ways that filibuster-busting reconciliation rules could be introduced into the legislative process and that the House wouldn’t necessarily have to first pass the Senate bill, which many House Democrats don’t like.

“There may be a rule in the House that allows us to pass both the Senate bill and reconciliation at the same time. There are a variety of mechanisms being talked about in meetings with leadership and with parliamentarians,” said the Maine Democrat.

Public option backers have an uphill climb ahead of them: White House Press Secretary Robert Gibbs declared the public option dead in a press conference earlier this week. And Senate Majority Whip Dick Durbin (D-Ill.), a public option supporter, told reporters outside the summit at the Blair House that the issue lacks Senate support.

The issue “might” come up during the rest of the day’s discussions, Durbin said, but “we’ve kind of been through this battle in the Senate … and you lose a handful of Democratic votes on the public option.”

But Adam Green of the Progressive Change Campaign Committee, who was on Thursday’s conference call with liberals, said Gibbs doesn’t have a real grasp of how much support there is for a public option.

“Gibbs, factually, was just not right,” Green said. “It is possible that he was getting overly absorbed in his bipartisan rhetoric in advance of today’s summit. … If [Senate Majority Leader] Harry Reid will actually hold a vote, we predict a lot of support.”

Green said to expect liberals to ramp up pressure on Democratic members to support a public option as soon as the summit adjourns. He specifically pointed to moderate Republican Sens. Olympia Snowe and Susan Collins of Maine as targets of their efforts, once the health care fight is over.

Liberals will be tweaking their message “from a ‘please’ posture to a more demanding posture in states where Members should have a backbone and represent the will of the people,” Green said.