Among many of his local congressional colleagues, the bets were that Rep. Norman Sisisky (D-Va.) would support the MX this year, despite the fact that he has always voted against building the missile in the past.

The moderate Democrat represents Portsmouth, which has a strong prodefense constituency because of its economic dependency on the Tidewater's shipbuilding industry. And Sisisky has backed the Reagan administration on most major defense initiatives.

In recent weeks, he had said that he was concerned about derailing the arms talks. Moreover, he returned from a White House meeting Tuesday saying he was convinced that President Reagan sincerely believed that the MX was a key to the arms control talks.

But shortly before noon yesterday, Sisisky, a member of the Armed Services Committee, decided he would vote against the MX.

"When you have a willing buyer, and you have a willing seller, and you can't make a sell, then something is wrong," Sisisky said.

"I am not convinced that the Soviet Union went to the negotiating table because of the MX," Sisisky said. "The Soviet Union went to the negotiating table because we have a $300 billion defense budget."

The only committee member who switched positions yesterday was Rep. Roy Dyson (D-Md.). The Eastern Shore congressman has opposed the MX in the past, but voted for it yesterday, saying that the arms talks were the "No. 1 issue of the times."

Dyson added, however, that his constituents seemed to care very little how he voted on the MX. He said that at the dozens of meetings he has attended in his district in recent days, the MX came up only if he raised it.

"It's sort of frustrating. You have these national issues of great importance [and] you'd kind of like to have some feedback from folks back home," Dyson said.

After reading that Dyson was thinking of switching positions, White House officials tracked the legislator down Monday at Patuxent Naval Air Center. The president was pleased, Dyson was told, and invited him to the White House.