The morning after President Reagan vetoed the farm bill, Rep. Roy Dyson (D-Md.) joined about two dozen other congressmen at the White House to listen to Reagan's pitch for the MX missile.

Dyson, who represents the rural Eastern Shore and southern Maryland, asked the first question: "How am I supposed to go back to the farmers in my distric Reagan has said that congressional failure to fund the additional 21 missiles would weaken the U.S. negotiating stance at the arms control talks in Geneva. But MX opponents contend that the system is costly, vulnerable and unnecessary.

Among those opponents are numerous residents of Prince George's County who frequently criticized Democratic Rep. Steny Hoyer at town meetings last year for his support of the MX. They told Hoyer that the billions of dollars could better be used for the poor and the elderly.

Hoyer said that he is undecided on how he will vote this year, adding that he hopes some sort of compromise can be worked out to buy the 21 additional missiles and defer the 48 missiles until the fiscal 1987 budget. Still, the pressure from liberals has not changed his basic philosophy about defense and the Soviet Union.

"I think there is a continuous analogy to be made to Munich, in that people thought if they gave Hitler more, then he would stop," he said.

"That's not the case," Hoyer said, adding that nations such as the Soviet Union will discuss arms control only when they perceive that it is the only way the United States will stop its buildup.

Sen. Charles McC. Mathias (R-Md.) also finds himself in a tough spot. As a Republican whose term ends next year, and one who is expected by many to seek reelection, Mathias must balance the White House pressure on Republicans to fall into line with the anti-MX sentiment in parts of Maryland such as Prince George's County.

Mathias, an outspoken arms control advocate, voted in 1983 to fund the first 21 MX missiles because of administration arguments that the weapon was important to arms control. But last year he lashed out against the missile system, saying that the MX had done nothing to further arms negotiations.

Mathias has not decided what he will do during this round of votes, according to a spokesman.

Like Mathias, Rep. James R. Olin (D-Va.) voted for the MX in the first vote in 1983 after hearing pleas that it was vital to arms talks. But he, too, switched positions when he decided that the MX had done nothing to further arms control.

Back in 1983 Olin was the newly elected congressman from Roanoke when he was invited to a small gathering in the Oval Office with Reagan, four other congressmen and a few aides. Olin had been critical of the MX during his election campaign. But when sitting a few feet from Reagan, he said, he found himself swayed by the president's arguments.

This year, however, despite attending the breakfast meeting with Reagan and despite lobbying by White House and State Department representatives, he is firmly opposed to the MX.

"The MX has a spotty record over the last 14 years," Olin said. "I just can't imagine the Russians make much of it one way or the other."

Besides, said Olin, his first vote in favor of the MX got him a lot of flak with his supporters back home.

"They said to me, 'We didn't send you there to vote for the MX. How could you vote for the MX?' " he recalled.

Most congressmen from Maryland and Virginia have supported the MX at one time or another. Of the 22 House and Senate members from the two states, only seven consistently have opposed the Reagan administration's MX plan.

Dyson is not the only one of those former opponents thinking of supporting Reagan this time around. Rep. Norman Sisisky (D-Va.) said that he is wavering in his opposition to the MX, although his skepticism about the missile system has not subsided.

"I'm still not satisfied, in my mind, that it's a good weapons system," Sisisky said. He said that the price tag bothers him, but so do accusations that killing the MX would derail the arms talks.

"It's a hard call, a difficult decision," said Sisisky, "but it will have to be proven to me that it [defeating the MX] would jeopardize the talks in Geneva."

Sisisky and Dyson said that even if they vote for the additional 21 missiles, they may oppose the 48 missiles for fiscal 1986.

Dyson explained that he is "still uncomfortable" about plans to deploy the missiles in old Minuteman missile silos, which he and other critics regard as vulnerable to a Soviet strike.

"The administration will still have to answer the question: Do you want to sink billions of dollars into a hole that's so vulnerable?" said Dyson.

"The battleground over the future of the program will be in the context of the 1986 MX program," agreed Sen. John W. Warner (R-Va.), chairman of the Armed Services subcommittee on nuclear weapons.

Warner, who attended the opening of the arms talks in Geneva last week, said he is "confident" that Congress will back Reagan on the 21 missiles at this crucial juncture in the arms negotiations.

"But there remains serious and conscientious concern by many," Warner said, "and those concerns will quite properly be debated in the [fiscal year] 1986 program."