Congressional Democrats are likely to seize on Mr. Buffett’s comments to bolster their argument that repeal of the estate tax amounts to a windfall for a few wealthy families. Republicans have pushed to eliminate the tax permanently or reduce the rate and exempt more estates by raising the value at which the tax takes effect.

Image Warren Buffett said on Capitol Hill on Wednesday that the estate tax helps keep the nation from becoming a plutocracy. Credit... Jason Reed/Reuters

Mr. Buffett said that in the last 20 years, tax laws have allowed the “superrich” to become richer.

“Tax law changes have benefited this group, including me, in a huge way,” he said. “During that time the average American went exactly nowhere on the economic scale: he’s been on a treadmill while the superrich have been on a spaceship.”

Lawmakers are under pressure to reach some agreement on the future of the tax because a law enacted by Congress in 2001 gradually phases it out through 2010, when it will be fully repealed for one year. The tax is scheduled to return in 2011 with a top rate of 55 percent on estates worth more than $1 million. For this year, individual estates valued at more than $2 million are taxed at a top rate of 45 percent.

The chairman of the finance committee, Max Baucus, a Montana Democrat, said yesterday that fewer than 1 percent of United States households currently pay the tax. He said repeal lacked support in the Senate and the purpose of the hearing was to solicit ideas for replacing the shifting rules and uncertainty of the current system.

Senator Charles E. Grassley of Iowa, the ranking Republican on the panel, said the estate tax should be repealed because “death should not be a taxable event.”