President Bush warned younger workers on Tuesday of a grim future for their Social Security benefits but critics accused him of exaggerating the retirement system’s problems to try and sell his plan to change it.

Bush used the word “bankrupt” five times at a White House forum to raise alarms about the financial state of Social Security decades into the future.

“By the time today’s workers who are in their mid-20s begin to retire, the system will be bankrupt,” he told an audience invited to a Social Security forum.

“So if you’re 20 years old, in your mid-20s and you’re beginning to work, I want you to think about a Social Security system that will be flat bust, bankrupt, unless the United States Congress has got the willingness to act right now.”

Rep. Sander Levin, a Michigan Democrat on the tax-writing Ways and Means Committee said Bush was just trying to scare Americans into supporting his plan to let workers shift part of their Social Security payroll taxes into private accounts.

“There is a challenge here,” Levin said. “There isn’t a crisis.”

The forum marked the start of an election-style campaign to try to promote Bush’s plan to overhaul Social Security, which has raised concerns among even among his fellow Republicans.

Bush offered no new details on how he would pay for the estimated $1 trillion to $2 trillion transition costs for establishing private accounts. Nor did he address concerns that the White House may favor curbing future benefit increases by changing the formula used to calculate them.

Senate Democratic Leader Harry Reid of Nevada said a leaked White House memo suggesting this showed Bush intends to break the “promise” to workers.

To highlight what he sees as the dangers facing the current program, Bush focuses on two dates: 2018 — when money paid out begins to exceed contributions –, and 2042, when the system’s trust fund is expected to run out of accumulated cash.

Critics say Bush’s use of the term “bankrupt” exaggerates the problem because even under pessimistic scenarios, workers and employers will still pay into the system and benefits, although reduced, would still be paid.

Andrew Biggs, associate commissioner at the Social Security Administration, told the forum a payroll tax rise from 12.5 percent to 16 percent would eliminate “forever” a projected $10.5 trillion financing gap. But he added, “That’s obviously a big increase that people wouldn’t like to pay.”

Sen. Lindsey Graham, a South Carolina Republican, has floated a proposal to raise the income level subject to Social Security tax from $90,000 now to as much as $200,000 to help pay for the transition costs of adding personal accounts.

But Bush has ruled out any increase in payroll taxes. That leaves him few options other than borrowing to pay the transition costs.

Treasury Secretary John Snow was in New York this week Monday for three days of meetings on Wall Street, where many investors are worried about such borrowing.

Vice President Dick Cheney, White House budget chief Josh Bolten and Council of Economic Advisers Chairman Greg Mankiw will also make appearances to pitch Bush’s plan.

Some Republicans are worried Bush’s drive to overhaul Social Security risks arousing voter anger and that may hurt them in the 2006 congressional elections. In an interview with the Wall Street Journal, Bush offered to “provide the political cover” to Republicans who support his plan.

Still, several influential groups have already come out against it. The AARP, a powerful senior citizens lobby, is spending millions to highlight its opposition to Bush’s plan.