WASHINGTON (AP) – Four years ago, Federal Reserve Chairman Alan Greenspan gave President Bush a crucial endorsement in the president’s drive to pass sweeping tax cuts. But it doesn’t appear that Bush will be as lucky when it comes to his ambitious plan to overhaul Social Security.

While Bush indicated for the first time a willingness to make high-income workers pay more Social Security taxes, Greenspan urged Congress to proceed with caution in setting up private accounts for younger workers.

Greenspan’s worry: that the billions of dollars in new government borrowing to pay the transition costs to the new system could upset financial markets and push up interest rates on everything from home mortgages to Treasury notes.

“If you’re going to move to private accounts, which I approve of, I think you have to do it in a cautious, gradual way,” Greenspan told the Senate Banking Committee on Wednesday.

Bush was asked in an interview before taking his Social Security pitch to New Hampshire on Wednesday whether he would oppose raising the current $90,000 limit on income subject to the Social Security payroll tax.

After weeks of administration officials addressing that issue only vaguely, Bush said he remained opposed to boosting the tax rate but left the door open to a possible increase in the amount covered.

“The only thing I’m not open-minded about is raising the payroll tax rate,” Bush said in interview Tuesday with regional reporters and published Wednesday in the New Haven (Conn.) Register. “And all other issues are on the table.”

In Washington, senators were eager to get Greenspan’s opinion of Bush’s Social Security plan.

Bush is proposing allowing workers born after 1949 to convert up to 4 percent of their income subject to Social Security taxes to personal stock and bond investments.

Currently, workers pay 6.2 percent of their income up to the cap of $90,000 into the Social Security fund and their employers contribute an equal amount.

The administration has estimated that the transition costs for the next 10 years would be $754 billion to convert to private accounts. Critics of the plan have said that vastly understates the true costs, which some estimate in the trillions of dollars.

Greenspan said the problem was determining whether the government’s increased borrowing needs would push up interest rates, and for that reason he said any changes should proceed “slowly and test the waters.”

The two sides in the Social Security debate saw different aspects of Greenspan’s testimony before the Senate Banking Committee as supporting their approach.

Republicans noted that Greenspan said he had long favored setting up private accounts as a way to address Social Security’s long-run financing problems. But Democrats said his insistence on a go-slow approach represented a lukewarm endorsement at best for Bush’s overhaul effort.

“There were lots of caveats,” said Sen. Charles Schumer, D-N.Y. “This was not like the ringing endorsement of the tax cuts in 2001.”

Because of the respect the 78-year-old Greenspan commands on Wall Street, his endorsement of an economic proposal is sought by members of both parties.

Bush cleared a major hurdle in his drive to enact tax cuts in 2001 when Greenspan endorsed the idea of cutting taxes, arguing at the time that the government’s projected budget surpluses were so large that tax cuts were a good idea. Those surpluses never materialized.

In his appearance Wednesday, Greenspan did endorse making a major switch in how benefits are calculated for workers when they reach retirement age.

Under one of the proposals put forward by Bush’s Social Security advisory panel, the level of benefits would be tied to increases in inflation, rather than increases in wages. Currently, retiring workers get about 40 percent of their wages replaced by Social Security. However, switching to an inflation index could cut that amount roughly in half.

Greenspan, who in the past has said benefit cuts will have to be part of the solution to Social Security’s problems, called the switch in indexing “one of the most effective ways to come to grips with closing the … gap between expected revenues and expected benefits.”

All last year, Greenspan used various appearances before Congress to push for action to deal with the impending retirement of 78 million baby boomers, saying the government had promised more than it could deliver not only in Social Security but also in Medicare, where the funding shortfall is even more severe.

In 1983, Greenspan chaired a Social Security commission that came up with a compromise plan that raised payroll taxes and trimmed benefits to deal with a funding shortfall the system faced at that time.

He said Wednesday that his commission had been successful because he and other panel members had worked closely with then-President Reagan and Democrats in Congress to make sure that whatever was adopted would be acceptable to both parties.