White House officials and Senate Republican leaders yesterday urged that critics of President Reagan's new nuclear arms reduction plan hold their fire until there is more time to assess the program and Soviet reaction to it.

In Washington, Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) said he was disappointed at the immediate "political sniping" at the president's proposal, which was unveiled Sunday, and that the plan had "so quickly become a partisan issue."

The sharpest domestic criticism thus far has come from former secretary of state Edmund S. Muskie, who was asked by Senate Minority Leader Robert C. Byrd (D-W.Va.) to give the Democratic response to the president's call for big reductions in Soviet and U.S. warheads and missiles.

Paradoxically, Reagan's plan got a boost yesterday from Muskie's former boss, Jimmy Carter. The former president, speaking with reporters in Copenhagen during a European tour, said, "Any time our nation comes forward with an offer of reductions of any substantial size, it's a good move toward peace and better understanding. I think it's an excellent proposal."

In Chicago, White House officials traveling with Reagan yesterday also sought to deflect criticism of the new proposal that appeared to be coming from Moscow. On Sunday, a commentary in the Communist Party newspaper Pravda by Defense Minister Dmitri Ustinov warned that Moscow would not allow what they view as the "existing balance of forces to be disrupted."

But Robert C. McFarlane, deputy White House national security adviser, said that the Ustinov commentary was written before Reagan wrote to Soviet leader Leonid Brezhnev last Friday outlining in very general terms what he was going to say in his Sunday speech in Eureka, Ill., proposing new strategic arms reduction talks.

"I really don't think this was their thoughtful criticism," McFarlane said. "I don't think it was a response." American officials have said privately that while they expect Moscow to offer sharply different counterproposals, they do expect that the Soviets will agree to begin talks. President Reagan, in his speech, said the United States would "carefully consider all proposals made by the Soviet Union."

The Soviet Union, however, has not yet formally responded to the president's call for talks to begin late next month in Geneva. Sources said that the letter to Brezhnev was basically a message proposing dates for the talks and did not contain as much detail about the proposals as Reagan announced in his speech. These sources also said Moscow had not yet been given the American proposal.

Reagan called for a two-phase reduction in atomic arms. The first phase is aimed at reducing by at least one-third the number of warheads aboard each side's missiles and achieving a sizable cut in the number of missiles themselves. The number of warheads allowed on the most accurate and threatening land-based missiles would also be limited to half of the reduced total. The second phase would involve further reductions in the lifting power, or throw-weight, of the respective missile forces.

The Reagan plan would require the Soviets to give up far more in terms of their land-based-missile striking power than the United States.

Yesterday, officials in Washington were unable to say with certainty how this two-phase proposal would work. A number of officials said they believed that if the United States could get a first-stage agreement along the lines laid out by the president, then the administration would sign such an accord and try to move on to phase two. Another official, however, suggested it would depend on the Soviet reaction and might only involve an agreement on both phases.

The Reagan proposals, officials here say, also do not require the United States to give up the new MX and Trident II missiles, B1 bomber or cruise missiles. The plan is aimed at limiting numbers, with each side free to choose how to distribute the lower levels of firepower.

But McFarlane, under questioning yesterday, said that "everything is on the table," meaning potentially negotiable. He said Moscow is very concerned about some of these new weapons and "that's going to be a powerful incentive" for them to negotiate.