The Senate's most powerful Democrat said yesterday that the capital gains tax cut President Carter has strongly opposed "would probably be in order" to stimulate the formation and investment of capital.

Majority Leader Robert C. Byrd (D-W. Va.) told reporters there is "considerable sentiment" in the Senate in favor of a capital gains tax cut, and said "a resonable cut" might be a boon to the economy. He said he had "no particular figure in mind" when asked what size cut he would advocate.

Meanwhile, Treasury Secretary W. Michael Blumenthal said in Bonn that the administration would be willing to consider a capital gains tax reduction, despite its repeated condemnations of the idea, if it is less expnesive and more progressively structured than either of the proposals now before Congress.

Blumenthal told The Washington Post that "to be acceptable," a capital gains relief proposal "must be more progressive more focused, be less expensive and do something for the homeowner."

He added that its drain on the treasury "would have to be well under $1 billion."

Blumenthal stressed that Carter "is not making any proposals to reduce capital gains. In fact, he doesn't want to reduce them at all. But I told him there will be some reductions, and I'm arguing that some cuts might actually be useful. So the president in effect said, 'Okay, let me see what they are.'"

Capital gains are profits from sale of assets, such as stocks and real estate, held for at least one year.

Currently, they are taxed at a maximum effective rate of 49 percent. Rep. William A. Steiger (R-Wis.) has proposed slashing it to maximum of 25 percent, which was the case before 1969.

Carter lashed out at the proposal as one that would benefit "almost exclusively the very wealthy."

As a compromise, Rep. James R. Jones (D-Okla.) has introduced a proposal to cut the rate to 35 percent. Jones' proposal was part of a package of tax cuts that included a few of the changes Carter has sought.

Proponents of a tax cut argue it would provide incentives for stock market investment and relief for some homeowners, but concede that the most immediate relief would go to upper-income taxpayers.

Opponents contend the Steiger proposal, which would cost the government $2.4 billion in lost revenue, and the Jones compromise, at $1.3 billion, would contribute to speculation and provide only meager benefits to low- and middle-income taxpayers.

Blumenthal said he has discussed with members of the House Ways and Means Committee "ideas" of how to make capital gains rate changes acceptable to the administration.

He emphasized that Carter has made no commitment to back his point of view. There is opposition in the White House to the concept of a capital gains tax reduction, and Vice President Mondale has shown little enthusiasm for the idea.

The treasury secretary noted that if the maximum rate were cut to just over 39 percent, only $40 million to $50 million would be lost.

"We'd have no difficulty in accepting a cut in the top rate to 39 percent," he said. "But to drop to the 35 percent [of the Jones bill], you'd have to invade the minimum tax provisions seriously.

"To invade that to help the commodity or real estate speculator doesn't make sense."

Byrd's statement came at a time when support for some form of capital gains relief has grown overwhelming in the Senate. A companion version of the Steiger bill, introduced by Sen. Clifford P. Hansen (R-Wyo.), has picked up at least 61 cosponsors.

On other matters, Byrd said the harsh sentence meted out to Soviet dissident Anatoly Scharansky on Friday has created an atmosphere in which it would be impossible to secure Senate approval of a strategic arms limitation treaty with the Soviets.

Scharansky was convicted of treason by a Soviet court and sentenced to 13 years at hard labor.

"If the Soviet government cannot treat its own citizens with fairness, then it serves to increase doubt on the part of those of use who have to vote on a treaty - doubt and public distrust throughout our country with respect to any agreement that might be reached," Byrd said at his regular Saturday press conference.