President Reagan went on the radio yesterday to rally support for the MX missile, wrapping up a week in which he has increasingly pinched the Democrats in Congress between their longtime commitment to reduce nuclear stockpiles and their more recent desire to erase public perceptions that they are soft on defense.

The administration's campaign to link the coming votes on MX funds to the potential outcome of arms talks with the Soviets, which begin Tuesday in Geneva, once again has put the Democrats on the defensive.

Democratic leaders now are trying to formulate a political strategy that will allow them to vote against the 10-warhead missile while escaping the stigma on the defense issue that plagued them during last year's presidential campaign.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), fearful of the political fallout on the MX issue, has insisted that the Republican-controlled Senate take the first vote this year on the MX.

His desire to provide cover for his troops grows out of post-election polls showing that voters believe the Republicans are best able to keep America strong.

One poll, by Democrat William Hamilton, presented to House leaders and aides, showed 57 percent of respondents saying Republicans are better on defense, and 19 percent saying Democrats are better.

"It's not just the Hamilton poll," an aide to O'Neill said last week. "He's O'Neill concerned about it."

O'Neill says he will vote against the MX, but some anti-MX lawmakers fear that his concerns about the party's image may mean he won't work as hard to defeat it as he did in past years.

Rounding out a week of calculated salesmanship, Reagan put more pressure on the Democrats in his weekly radio address yesterday by calling the vote in Congress "a key test of American resolve."

Saying that what Congress does could "directly and perhaps dramatically affect the outcome" of arms talks with the Soviet Union, Reagan added:

"If the Congress acts responsibly, our negotiators will have a chance to succeed. But if we don't have the courage to modernize our land-based strategic missile systems, the Soviets will have little reason to negotiate meaningful reductions, and why should they?"

Sen. Gary Hart (D-Colo.), responding to Reagan, outlined the arguments some Democrats are using:

That the MX is too vulnerable a target to be a good bargaining chip; that in a time of budget austerity, it is a waste of money, and that the Reagan administration's $1 trillion military buildup already has sent a signal to the Soviets of American resolve.

"We can't show the Soviets we're strong by voting for a missile that's weak," Hart said.

"We can't buy needed military equipment if we waste our money on weapons that aren't needed. And we simply won't reduce the threat of nuclear war by building the MX missile."

MX opponents acknowledge that Reagan now appears to have the upper hand on the first round of votes this year. "This has been marketed superbly," one Democrat said. The Democrats are seeking to turn the debate into one on waste and budget austerity, casting the MX as a costly lemon that could be easily wiped out by Soviet missiles.

"The point we're making is that there's nothing strong on defense about deploying a vulnerable missile," an aide to one leading Democratic opponent of the MX said. "Maybe the outcome will depend on getting that point across."

But the MX opponents are struggling uphill against the administration argument that voting down the missiles now would undercut U.S. negotiators in Geneva.

That argument is one reason House Armed Services Committee Chairman Les Aspin (D-Wis.) reportedly indicated to House leaders last week that he is inclined to support Reagan in the first vote, scheduled the week of March 25. That vote will be on whether to "unfence" 21 MX missiles approved in fiscal 1985 for which the funds were frozen.

Aspin was a key to administration successes in past years, and MX opponents had hoped to turn him around this year.

MX opponents now plan to schedule a meeting of the House Democratic Caucus for March 20 and press for a resolution opposing the release of funds for the 21 "fenced" missiles, in part hoping to pressure Aspin to vote with them.

Aspin is one of several Democrats who have supported the administration in the past, but who now say they want to review the program in the fiscal 1986 budget debate later this year.

The administration has asked for an additional 48 missiles in its new budget proposal. Congressional sources indicated that Aspin likely will seek reductions in the administration's request.

On Wednesday, Aspin met with Sens. Sam Nunn (D-Ga.), Albert Gore Jr. (D-Tenn.), William S. Cohen (R-Maine) and Rep. Norman D. Dicks (D-Wash.).

The five were instrumental in helping the administration on earlier MX fights.

Two members who attended said the meeting was "inconclusive" while indicating that they were "inclined" to back Reagan on the "unfencing" vote in two weeks.

But they said they intend to look more critically at the MX program in the fiscal 1986 defense bill.

"We need MXs out there," Nunn said. "But that's not the same as saying we've got to give the Gipper one as he goes to Geneva."

Nunn said he has questions about the MX, ranging from the missiles' vulnerability to the need for the full 100 missiles and 123 spares the administration wants. He said he fears that building all 100 missiles could move the United States toward a "launch-on-attack" posture. Dicks said he and Gore had told the administration earlier that they wanted to review the program once 50 missiles had been approved, and he indicated that 42 was close enough to 50 to suit him.

"My own view is that in 1986, we ought to give MX a de novo review, top to bottom," he said.

Nunn expressed the same attitude, saying "I'm not focusing as much on 'fencing' as I am on 1986."

That approach, however, has helped give the administration momentum in its campaign to win the first test vote of 1985. After that, it may become more difficult for MX opponents to shut down the program or substantially reduce it.

Reagan's speech from Camp David, where he is spending the weekend, followed a week of intensive lobbying. He spoke to more than 100 members of Congress during the week, and White House officials expressed optimism Friday that the missile program would survive in the Democratic-controlled House. But a White House official said that the vote was still "too close to call" in the GOP-controlled Senate despite Reagan's efforts. Officials said that legislative strategist Max L. Friedersdorf spoke to several key senators and reported afterward that the MX remained in jeopardy.