Yesterday morning, Sen. Charles McC. Mathias (R-Md.) still had not decided how to vote on the MX, despite intense lobbying by both sides and more than 500 phone calls on Monday, largely from opponents of the missiles.

But yesterday morning, the state's senior senator, who before the vote was considered one of the prime undecided senators on the measure, visited the White House for a short discussion with President Reagan.

Then around noon, Mathias rode back to Capitol Hill with Reagan in the president's limousine. Mathias said he requested the meeting because he still had a few nagging questions. He declined to reveal the questions, but said Reagan answered them to his satisfaction.

By the time he stepped out of the president's car, he had decided to vote for the missile system.

As a Republican, whose term ends next year and who is expected to run for reelection, Mathias must balance the apparently strong anti-MX sentiment in Maryland with pressure from the Reagan administration on his party's senators.

Mathias, who voted with the Reagan administration on MX in 1983 and against it in 1984 because of the failure of the arms negotiations, said he had been leaning in favor of the MX since the weekend. Even his floor statement kept some spectators guessing.

He began by dismissing several administration arguments: "The so-called bargaining chip is not overly persuasive . . . . Experience teaches that the bargain in mind is with Congress and not with the competition."

"Another concern expressed is that the Russians will view a vote one way or another as a signal of aggression or a sign of weakness. My guess is that they are so preoccupied by the Strategic Defense Initiative that they won't spend too much time analyzing our MX vote."

But Mathias said he was voting for the additional 21 missiles because he believed they would improve deterrence.

"It was a very difficult decision," he said after the vote.