WASHINGTON – President Bush yesterday opened his campaign in Congress for historic changes in Social Security, asking for legislation that will let workers create private retirement accounts within the government-run program.

Skeptical Democrats told the president any legislation must be bipartisan to prevail, according to participants in a session that signaled the likely start of a fierce struggle.

While the call for private accounts is the most controversial element of Mr. Bush’s vision for a remodeled Social Security, the matter is bedeviled by difficult issues.

Even before lawmakers sat down for the White House meeting, Scott Mc-Clellan, the presidential spokesman, indicated Mr. Bush favors government borrowing rather than tax increases to cover the costs of “transition to a better Social Security system.” At a time of record deficits, estimates of the money needed range from $1 trillion to $2 trillion or more over a decade. Even so, said Mr. McClellan, “Today’s system is unsustainable. Younger workers are facing massive tax increases or massive benefit cuts if we don’t act.”

Republican officials said after the meeting that no timetable has been set for action in either house of the new Congress that meets in January. But supporters are eager to begin. Rep. Jim Kolbe, a Republican of Arizona, scheduled a news conference for today to introduce legislation giving workers the choice of a private account.

Several officials said Rep. Allen Boyd, a Democrat of Florida, would announce his support for the measure, giving it a bipartisan flavor. “I’ll be at the press conference,” he said.

Mr. Bush “said several times he ran on this” issue during his successful campaign for a second term, said Rep. Bob Menendez, a Democrat of New Jersey who attended the White House session. He quoted the president as saying he “intended to spent a lot of his political capital” on it.

Many Republicans support changes along the lines Mr. Bush advocates as part of a bill to solidify Social Security’s financial foundation. Many Democrats oppose private accounts as a threat to the program that provides a monthly retirement check to millions of Americans. But Democrats at the meeting, holding their fire, said they would await more information from the president. “I’m willing to wait and see the numbers that they produce,” said Rep. Charles Rangel of New York.