Bennie Thayer, the Prince George's County businessman who headed Jesse L. Jackson's presidential campaign effort in Maryland, has said for months that if there is one issue that makes Rep. Steny Hoyer (D-Md.) vulnerable in Prince George's County, it's his past support of the MX missile.

Hoyer agrees, yet this week he voted, again, in favor of the MX, siding with the majority in the House of Representatives to release $1.5 billion to build 21 more missiles.

"I think it would have been politically much easier to vote against it," said Hoyer, who received hundreds of phone calls from constituents opposed to the MX before the vote Tuesday. "But I happen not to think that's the right vote."

Thayer, who has said that black activists in the Democratic Party might consider running a candidate against Hoyer in 1986, said many in the more liberal wing of the Prince George's Democratic Party believe "that [MX] money could be better allocated on the domestic side for the underprivileged."

Hoyer said he is aware of that sentiment. "That made it a more difficult vote," he said.

President Reagan lobbied hard for the funds for the MX, a nuclear missile he insists is instrumental to his efforts to get the Soviets to negotiate on arms. But opponents charge that the weapon is not needed and its price tag is too high during a time when Congress is facing stiff domestic budget cuts.

Hoyer's vote not only put him at odds with part of the Prince George's County Democratic Party, but also with the House Democratic leadership. But Hoyer, who often is aligned with conservatives on defense issues, said he felt he had to vote for the additional 21 missiles.

"I believe it is useful that we go into the negotiations with a unified front and with our principal negotiator, that is the president of the United States, able to have a consensus support behind him on an issue that he says is critical to those negotiations," Hoyer said. But he added that he is aware of the realities of budget politics, and that he is wary of casting another vote in favor of the missile system this year.

He has advocated deferring a vote on the next 48 missiles until next year. The Reagan administration wants 48 new missiles approved by Congress this year in the fiscal 1986 budget.

Hoyer said he spoke to several members of the Democratic leadership about his support of the MX, but when asked if they applied "considerable pressure" on him to change his vote, he chuckled: "I've been at this a long time now. I don't really think you can say that I came under considerable pressure, not compared to some times in the Maryland Senate . . . . "