Senate Democrats presented President Bush with a list of conditions for their support of a compromise Medicare (search) prescription drug bill on Tuesday, demanding an ironclad guarantee of a government benefit for the elderly in areas where no private drug plan is available.

The letter, signed by more than two dozen Democrats, also rejected a conservative-backed provision contained in the House-passed bill to inject greater competition into the health care program for seniors.

At the same time, the letter signaled acceptance of one important change in policy -- making Medicare coverage dependent on the income of the beneficiary. The House bill included such a provision and the Senate indicated support for the concept without formally agreeing to it.

"We understand that hard choices and compromises will be necessary," the Democrats wrote, but pledged to oppose any bill that fails to address a half-dozen conditions they laid out.

While couched in diplomatic terms, the letter underscored the challenge facing majority Republicans and the White House as they begin seeking agreement on an issue that has defied compromise over the years.

"Don't be deceived by the overwhelming margin here," said Democratic Leader Tom Daschle (search), referring to the 76-21 vote on legislation that cleared the Senate last month.

"We have some very significant concerns about the prospects of reducing the commitment to seniors and providing the kind of meaningful benefit that we all have proposed in this legislation," he said.

Democrats laid down their legislative markers as Sen. Charles Grassley, R-Iowa, conceded it would be difficult to achieve a final agreement before the end of the month. The House plans to begin a lengthy vacation in two and a half weeks, the Senate a week following. Once gone, lawmakers won't return to the Capitol until after Labor Day.

Both houses approved legislation in June to give the elderly a government-backed prescription drug benefit, with large subsidies for lower-income beneficiaries. Both bills also would call for greater competition in the system, largely through the introduction of private managed care plans that seniors could choose over their traditional government-run health care benefit.

While the Senate bill cleared comfortably, House GOP leaders wrote a measure that squeaked to passage by a single vote, largely along party lines.

"I do think that they've got a major problem over there," Grassley said. "They've got people on the left that want something different than people on the right, and it's difficult to get a compromise."

In their letter, initiated by Sen. Edward M. Kennedy (search), D-Mass., as well as Daschle, Senate Democrats said they would oppose to provisions that were included in a House measure that passed on a near party-line vote. One would require traditional Medicare to compete directly with private insurance beginning in 2010.

"It is wrong to legislate a vast social experiment that would raise premiums for Medicare and victimize the oldest and sickest senior citizens," they wrote.

Democrats also expressed opposition to a House-passed provision that would create tax-advantaged savings accounts to be spent on health care. The idea has long had support among Republicans, and Grassley, in his comments to reporters, expressed backing for it.

Additionally, they wrote, "There must be a backup Medicare plan in areas of the country in which private ... prescription drug plans are unwilling or unable to provide the drug benefit." These protections, they added, must provide "no less protection than in the Senate-passed bill."

Democrats also called for protection for low-income senior citizens, provisions that both bills have, and called for innovative steps to hold down drug costs while making lower-cost generic drugs more readily available.

They urged for steps to discourage employers from dropping the prescription drug coverage they currently provide their retirees.