Mr. Bush said Wednesday that a deal on overhauling Medicare was "within reach" and urged "America's seniors to speak up, to call and write your representatives to urge them to work out a final bill."
Speaking at the White House, the president said Congress needed "to act quickly, to act this year, not to push this responsibility to the future." He said that while "a few difficult issues remain, the Congress has made tremendous progress and now is the time to finish the work."
Negotiators from the House and Senate are trying to work out differences between the bills each house passed in June.
But some of Mr. Bush's Republican allies are worried that the current proposal will go over Mr. Bush's $400 billion spending limit.
While no formal cost estimates have been prepared by congressional budget experts, legislation exceeding the limit could send congressional negotiators back to the bargaining table before they could seek passage in the House and Senate.
"Sure, it's a concern that the first draft might be bigger," Sen. Charles Grassley, R-Iowa, said. "But in the end it can't be bigger."
Three dozen anti-tax and conservative groups have written House Majority Leader Tom DeLay, R-Texas, that Congress should scrap the drug proposal because it is certain to cost far more than $400 billion.
And a nonpartisan group that works to protect the rights of retired people began airing a television commercial Tuesday urging people to ask lawmakers to reject the drug benefit plan. The ad, paid for by the Alliance for Retired Americans, claims that 4 million retirees would lose the prescription drug coverage they receive from their former employers if the legislation being crafted becomes law.
In bargaining sessions that went late into Tuesday night, lawmakers neared agreements to ease access to lower-cost generic drugs, jettison co-payments for home health care visits and shelter hospitals from $12 billion in cuts.
At the same time, lawmakers worked toward a compromise that would result in savings of as much as $10 billion over a decade from a program that reimburses doctors who provide certain drugs to battle cancer and other diseases.
Grassley said negotiators would begin voting on issues Wednesday, including the reimbursement for cancer doctors.
With Congress aiming to finish this year's work in November, time is fast running out to wrap up the details of this complex legislation.
Key issues remain unresolved, including calls for direct competition between traditional Medicare and private health plans.
Other disputed issues are limits on spending if future cost increases are larger than expected, tax-preferred savings accounts that people could use to meet their medical costs, and importation of brand-name drugs from Canada and other countries, where they often sell for much less than in the United States.
Officials said informal discussions on the last issue included the possibility of a trial period for importation of drugs from Canada under Food and Drug Administration supervision. But Health and Human Services Secretary Tommy Thompson, leaving the talks Tuesday night, said, "We still believe safety is a big issue."