On the afternoon of Feb. 27, Paul C. Warnke, director of the Arms Control and Disarmament Agency (ACDA) in the Carter administration, called on Sen. Arlen Specter (R-Pa.).

Warnke urged Specter to stand firm in his opposition to the MX missile, and what he told the first-term senator had a powerful impact on him: it backfired.

He told Specter that the MX was worth building only to bargain away and noted that President Reagan had said he will not bargain it away. Then he told a story about cancellation of the B1 bomber during the Carter administration.

Warnke told Specter that he had received word of President Jimmy Carter's decision only about 15 minutes before it was announced. He was disappointed, he told Specter, because he wanted to get something from the Soviet Union in the strategic arms talks in return. And when he expressed that disappointment to his Soviet counterpart, the Soviet negotiator said he had the same reaction. He had wanted to get some credit back home for forcing the United States to yield on the new long-range bomber.

Just after 5 p.m. yesterday, Specter caught the subway to the Capitol. Not even the Reagan administration, which was lobbying for the MX, was certain what he would do.

How are you going to vote? Specter was asked.

"I'm going to vote for it," he replied softly. It was what Warnke had told him that influenced his decision, he added without irony. Specter concluded that the United States should try to get something for the MX rather than unilaterally cripple the program.

"I'm terribly sorry I ever spoke to him because Sen. Specter got it wrong," Warnke said last night. "It makes no sense to go along with the idea that building the MX will scare the Russians."

The road that carried Arlen Specter from his vote against the missile last year to his decision to support President Reagan's MX request yesterday was filled with unexpected twists. The final one involved a confrontation with the president at a luncheon yesterday in the Capitol over White House threats of political retaliation against GOP senators who opposed the weapon.

Specter made up his mind less than two hours before the vote, he said, and in the end, the current arms talks in Geneva proved critical, just as the Reagan administration had calculated when it decided to schedule the first MX vote of 1985 a week after the resumption of those talks.

But to get there, Specter endured months of lobbying from the White House and an equally determined campaign from MX opponents in Pennsylvania. After he voted against the missile last year, Defense Secretary Caspar W. Weinberger called to say the administration had new information that might persuade Specter to support the missile. After that, Specter opened his door to all callers.

He went through briefings from Defense Department and National Security Council officials, read through reams of technical documents, went to the White House for a session with the president, took phone calls from here and abroad from supporters of the administration, and was pressed by a coordinated, grass roots campaign of letters and meetings and phone calls from antinuclear activists at home.

But all the months of work by the administration almost went for naught when last week, Paul Michel, Specter's administrative assistant, got a phone call from Mitch Daniels, a new member of the White House political operation.

Daniels was calling his friend Michel to pass on a message: when the White House staff sat down to allocate Reagan's time in helping vulnerable GOP senators in fund-raising and other campaigning, friends would come first.

The message was somewhat oblique, but later in the week it confronted Specter head on when The New York Times published a story saying the White House was pressuring GOP senators on the MX and threatening retaliation. Specter was singled out in the piece by an unnamed White House aide, who said the president was not inclined to go to Pennsylvania to raise money for Specter if the Pennsylvanian was voting against the White House on key issues.

Specter was furious. At a meeting Friday night in Philadelphia with a coalition of MX activists, he denounced White House aides and said he was so mad he had to "go to the steam room to cool off."

Yesterday, when Reagan came to lobby senators at the Capitol, Specter questioned him about it directly. "I told him I thought it was very destructive and very debilitating for senators to have that happen," Specter said.

But other forces were pushing Specter to support the administration.

Around Thanksgiving 1983, he had gone to Europe to study the nuclear issue. In Geneva, U.S. arms negotiators arranged a meeting for him and Sen. Carl Levin (D-Mich.) with Victor Karpov, now the chief Soviet arms negotiator.

"It was like talking to a wall," Specter recalled.

The meeting was long and difficult. At one point, according to a Specter aide, Karpov blew up. "If I lived in Pennsylvania, I wouldn't vote for you," he said to Specter.

"If you lived in Pennsylvania, at least you could vote," Specter shot back.

Yesterday, as he walked into the Capitol, Specter said the meeting with Karpov had influenced his decision to vote for the MX. "They walked out on the talks -- and then they came back," he said.

Back in Pennsylvania, the pressure mounted. Specter went home for a series of town meetings last week. In an American Legion Hall in Vandergrift, Pa., 11 people had driven from a nearby town to push him to vote no. Common Cause in Washington reached more than 4,000 of its members in Pennsylvania and urged them to contact Specter directly.

Yesterday morning, two receptionists in his office tried in vain to handle the calls coming in overwhelmingly urging him to vote against the MX. In the first few hours of the day, his Philadelphia office recorded 468 calls opposing the MX and 65 supporting it.

Still, the threats of political retaliation nagged at Specter.

On Monday, chief White House lobbyist Max Friedersdorf tried to reach the senator, but with Specter in Pennsylvania, passed a message through Paul Michel. There will be no retaliation, he said. Vote on the merits of the issue, not on your reaction to the stories about the threats.

Specter's questioning of Reagan was one of the dramatic moments of yesterday's luncheon in the Capitol. "He's mad," one White House official said afterward, and they continued to keep Specter on the list of undecided votes.

For the next several hours, Specter holed up in his third-floor office, finally deciding about 3:30 p.m. to vote for the missile.

"So that there will be no doubt about my motivation on my vote," he said in his dictated statement, "I shall not have President Reagan come to Pennsylvania or elsewhere to help me raise campaign funds.