copyright 2009, The Washington Post WASHINGTON -- With President Obama poised to give a health-care address Wednesday before a joint session of Congress, administration officials promised that he will deliver a detailed prescription for reform despite the risks of spelling out exactly where he stands. Vice President Joe Biden, in an appearance in Washington on Thursday, said the speech will map out 'in understandable, clear terms what our administration wants to happen with regard to health care, and what we are going to push for specifically.' Though favoring one proposal over another carries political risks, potentially limiting what Obama might be able to claim as a victory, senior administration officials said the speech will satisfy demands that he clarify which provisions he supports and which he could jettison. The contents of the speech are largely decided, officials said. 'I don't think that there will be any ambiguity about where he thinks we have to go from here,' senior adviser David Axelrod said. Joel Benenson, the lead pollster for Obama, said in a memo to Democrats on Capitol Hill that support for a health-care overhaul is higher than it appears and will increase once the specifics are made clear. 'There is little doubt that the moderate numbers of support for the president's health insurance reform plan are based in large part on a lack of awareness of the details of the plan,' Benenson wrote. Officials would not elaborate on what Obama will say -- namely, whether he will continue to advocate for a government-run insurance program. But advisers said Obama will address the question -- setting the stage for a showdown between liberal Democrats insistent on a public option and conservative Democrats and Republicans who oppose it. In the Senate Finance Committee, where negotiations on a bipartisan compromise are underway and a conference call is scheduled for Friday, lawmakers are trying to reach a deal by Sept. 15 with perhaps as many as three Republicans on board. The White House is focused on one of the three, Sen. Olympia Snowe of Maine, who they view as the Republican most willing to reach an agreement with the White House. On Thursday, aides to Snowe confirmed that the senator is talking with administration officials, particularly with regard to her 'safety-net fallback option.' Under that proposal, the government would sponsor a nonprofit insurance plan but it would become available only in states or regions where private insurance firms had failed to offer a reasonably priced product that would be affordable to 95 percent of the population. Snowe's spokeswoman, Julia Wanzco, said the senator 'has had an open line of communication with the White House over the course of the past few months.' Meanwhile, pressure from liberals is mounting. In a statement Thursday, House Speaker Nancy Pelosi, D-Calif., called a public option essential to real reform and said not including one would be 'a major victory' for the insurance industry.' In a letter delivered to the White House on Thursday, the two leaders of the bloc of House progressives told Obama they will not support a health-care plan without a public option -- and demanded a meeting to inform him face to face. 'Any bill that does not provide, at a minimum, a public option built on the Medicare provider system and with reimbursement based on Medicare rates -- not negotiated rates -- is unacceptable,' said the letter, signed by Reps. Lynn Woolsey, D-Calif., and Raul Grijalva, D-Ariz. 'A health reform bill without a robust public option will not achieve the health reform this country so desperately needs,' it continued. 'We cannot vote for anything less.' Also Thursday, two major liberal advocacy groups, MoveOn.org and the Progressive Change Campaign Committee, launched petition drives demanding that Obama endorse a public insurance option. Should Obama abandon the public option, progressives will come under tremendous pressure to back the plan anyway. White House advisers are confident that Democrats across the spectrum will be reluctant to block even a health-care plan that they think falls short, but the advisers said they recognize that the president must work to keep the party base satisfied if he embraces a more modest plan. Biden, in his appearance, predicted the administration is 'going to get something substantial' once the negotiations are complete. 'It's going to be an awful lot of screaming and hollering before we get there, but I believe we're going to get there,' Biden said. 'The president's going to lay out for you very clearly on Wednesday what he thinks those pieces have to be and will be. But that's as much as I should say, and the president will tell you a lot more on Wednesday.' Staff writers Ben Pershing, Michael D. Shear, Dan Eggen, Lori Montgomery and Mike Fletcher contributed. 