Congress should more than double tax rates for many hedge fund managers and private equity partners who classify their pay as capital gains, former Treasury secretary Robert Rubin said.

The Clinton administration official, speaking in Washington at a conference on tax reform hosted by the Hamilton Project, said fund managers are paid to perform services. Their share of future profits earned by managing other partners’s money, called “carried interest,” probably should be taxed at rates as high as 35%, like normal salaries, instead of the 15% rate for capital gains, they said.

“One very good argument is to be made for treating it as ordinary income,” Mr. Rubin, now chairman of the executive committee at Citigroup Inc., said. Fund managers are “basically performing a service,” he said.

The 15% tax rate paid on carried interest by many fund managers is attracting congressional attention as lawmakers search for revenue to pay for spending priorities at a time when many fund executives reap billion-dollar paydays for a single year’s work.

Mr. Rubin successor as Treasury secretary, Lawrence Summers, said he was also concerned that some fund managers are able to abuse tax laws to convert some forms of income that should be taxed at higher rates into capital gains.

The chief tax counsel for the Republican staff of the Senate Finance Committee, Mark Prater, said aides have been studying the tax implications of the structure for several months and will forge a “piecemeal response” to the issue.