Sen. Edward J. Mason (R-Cumberland) calmy put a motion before a small subcommittee here last week that left one Baltimore senator nearly apoplectic and one of his Montgomery County colleagues very worried.

Mason simply attempted to get the group to vote on one piece of his mass transit funding proposal -- a piece that ultimately could cost Baltimore $7 million a year -- when he thought he had the votes to pass it. The vote, however, was delayed, and when the issue came up Monday for its first test, Mason lost.

Still, the hubbub over this first mass transit skirmish showed just how important Mason has become this year to the fortunes of the Washington suburban delegations, whose major concern here is a stable funding source for the Washington Metro system.

Mason's proposal, several suburban legislators fear, could jeopardize a plan proposed by Gov. Harry Hughes to provide 75 percent state funding for Metro's operation and 100 percent funding for Baltimore's subway.

"If Mason tries to make this a fight, we could end up tied up in knots on the Senate floor," one Prince George's senator observed this week.

Mason's plan would provide 75 percent funding for both the Washington suburban and Baltimore systems -- leaving the local governments with a 25 percent share. It is a proposal the Baltimore legislators violently oppose.

So Mason has suddenly found himself at the center of the mass transit maelstrom, and the bespectacled western Maryland Republican is still wondering how he got there.

The stage was set last summer when the governor, the House speaker and the Senate president named him to head a special committee studying the critical question of providing funds for road construction and mass transit operations.

Mason, a rural senator who was one of the leaders of an antisubway filibuster in the Senate several years ago, says he has wondered himself why he was named to chair the committee.

"Maybe they thought if they could get me to accept chairing it . . . it would put me in a position of supporting its report," Mason said last week. "But now they probably wished they hadn't named me."

In fact, Hughes incorporated many of the Mason committee's proposals in his own transportation package, but Mason said that the Hughes' plan fails to address the "long-range problems of transportation. His biggest mistake was not asking the Baltimore region to pick up 25 percent of the (subway operating) deficit like the Washington area must do."

Perhaps a measure of the respect Mason enjoys from his colleagues was highlighted by the fact that Sen. Victor Crawford (D-Montgomery), who vehemently opposes Mason's plan because he fears an adverse affect on Montgomery, acknowledges the plan has "equity and fairness on its side."

"But on the big Metro bill, we need Baltimore on our side so desperately that we'll have to go along with them against this," Crawford said.

Mason realizes he is waging an uphill battle with the suburban Washington and Baltimore delegations against him, but he is used to tough fights as a member of the Senate's rural minority and leader of the tiny Republican contingent in the 47-member body.

Mason, who was elected to the Senate in 1970 and as minority leader in 1975, said he is accustomed to being "constantly outnumbered."

But at least one of his colleagues refuses to write him off in this fight, saying, "He is smart, he's respected and for a Republican he has a certain amount of clout."