Sen. Charles Mac. Mathias, who has teetered on the left edge of the Republican Party for much of his two decades in Congress, may finally fall off this November when it comes time to vote for president.

While seeking a third term in a campaign almost totally devoid of partisan rhetoric, Mathias refuses to say for whom he will vote if independent candidate John B. Anderson is on the ballot in Maryland. He also has declined to serve on a state advisory committee for Republican nominee Ronald Reagan, failed to show up at a Reagan rally in Silver Spring where he had been expected, and avoided any mention on billboards and campaign literature of his membership in the Rebuplican Party.

In a two-way race between Reagan and President Carter, Mathias says he will vote for Reagan. But that is not information he readily volunteers. In four speeches last week, Mathias managed to make no mention his party's nominee for president.

Only when pressed will Mathias say that he will vote for Reagan over Carter. Asked why, Mathias replies: "Last January I said I would support the nominee [of the Republican party] and I am prepared to do that."

What if a three-way race, including his Congressional friend and fellow backsliding Republican, Anderson?

"I am very doubtful we will have the option of supporting John in Maryland," is the artful response. "I don't think his name is going to be on the ballot." (Anderson currently is on the ballot in Maryland, but the matter is now being appealed to the Fourth U.S. Circuit Court of Appeals, which is expected to rule next month).

What if Anderson wins the court challenge?

"Then," says the sentor, "I'll have to face that situation then."

Mathias, as the state's senior and highest-ranking statesman, apparently has decided that the time has come to rise above politics.

In two speeches in Ocean City last week, he didn't even bother to tell his audiences that he is a candidate for reelection. And only if he is called on to do so does Mathias admit to being a member of the Republican Party.

The reason for all this is not just that Democrats outnumber Republicans in Maryland by about two-to-one. Mathias has long demonstrated that he can be elected as a Republican. But this year, his party has taken a sharp turn to the right, away from the middle-of-the-road principles that have permitted the GOP to encompass a Mathias, or a Jacob Javits of New York.

Mathias had been expected to appear at a Reagan rally in Silver Spring on Sept. 6. More than 200 supporters of the Reagan-Bush ticket turned out for "Commitment '80," one of 486 rallies throughout the country. An hour before Reagan addressed the audience iva closed-circuit television, Reagan activists were touting the imminent arrival of Mathias. But shortly before the program began, a Mathias aide showed up and explained that his boss had a speaking engagement in Ocean City.

At a forum in Baltimore last Thursday, in the first face-to-face encounter with his Democratic opponent, State Sen. Edward T. Conroy, Mathias was asked what he thought of the Republican platform. His answer touched only on planks with which he disagreed: abortion, the Equal Rights Amendment and the selection of federal judges.

Nowhere in his campaign literature, or on the billboards across the state that urge voters to "Re-Elect Mac Mathias," is the word Republican mentioned. Mathias even turned down an invitation by Reagan to serve on his 20-member Maryland advisory committee, citing membership on the special Senate committee investigating Billy Carter as the reason for declining.

What would Mathias tell a voter who asked him why he should vote for Reagan instead of Carter? The graying patrician from Frederick is silent for several moments. Then, measuring his reply carefully, he offers this:

"I have inquired very carefully about Reagan's performance as governor in California, among liberals, moderates and conservatives alike, and they agree that he was a pragmatic, objective governor. He appointed competent people and allowed them room to do their jobs. He didn't do everything the way I would have done it, but . . ."

Of the Democratic nominee, Mathias says only, "Competence has not been the hallmark of the Carter administration."

As for Mathias' reasons for declining to serve on Reagan's Maryland advisory group, the senator explained that he "didn't want anyone to interpret my actions on Billygate as motivated by wanting to help Reagan," as unlikely as that might be, given his reluctance to associate himself with Reagan.

Don Devine, Reagan's Maryland chairman, supports Mathias's explanation, adding that two other senators, Republican Paul Laxalt and Democrat Edward M. Kennedy, turned down asignments to the Billygate committee because they didn't want anyone to misinterpret their motives -- Laxalt because of his close association with Reagan and Kennedy because, at the time, he was challenging Carter for the Democratic nomination.

Another issue on which Mathias is somewhat less than emphatic is the matter of how he would vote if Republicans should gain control of the Senate this November. A GOP-controlled Senate would likely result in the elevation of several arch-conservatives to the chairmanships of major committees: Strom Thurmond of South Carolina on Judiciary; John Tower of Texas on Armed Services; Jake Garn of Utah on Banking, and Jesse Helms of North Carolina on Agriculture, among them.

Mathias' first reaction to the question of whether he would vote for these men, with whom he often disagrees philosophically, is a quizzical look, followed by a rhetorical question: "Do you know what I would be if that [the Republicans took control of the Senate] happened?" The answer, he quickly volunteers, is "chairman of the antitrust subcommittee "[Of Judiciary]." Mathias is the second-ranking Republican on the committee, and because the chairman traditionally does not also chair a subcommittee, Mathias would have his pick of them.

"I could look forward to that very serious responsibility," he muses. As far as former Dixiecrat Thurmond replacing Kennedy as head of the committee, Mathias shrugs and suggests, "the chairman is not the be-all and end-all." Furthermore, says Mathias the defeat of his friend and fellow liberal, Jacob Javits, in the recent GOP Senate primary in New York, makes a Republican takeover of the Senate less likely.

At the initial debate of the Maryland campaign, sponsored by two liberal political clubs in the northeast Baltimore and held at the Faith Presbyterian Church, Mathias' liberalism was the target of organized opposition in the audience. Members of an ad hoc organization called Citizens for Good Government bombarded Mathias with critical statements about his votes in favor of the Panama Canal "giveaway," creation of the new Department of Energy, aid to the "new Marxist government of Nicaragua," and federal funding of abortions; and his votes against requiring prayer in the schools and taking money from schools that teach sex education.

As Mathias left the debate, he was greeted outside by half a dozen people holding up small wooden crosses to which cardboard cutouts of a human fetus had been nailed.

"Baby-killer," muttered one woman. "Communist," called out an old man. Another woman wlaked up to the senator and whispered "traitor." Mathias reacted by shaking his finger in her face and warning, "Look very deeply into your heart before you say that of anyone."

Conroy was applauded vigorously by those same people when he told the audience, "I very much disagree with the abortion plank [in the Democratic platform]."

Like Mathias, Conroy supports ERA, saying he was the "prime sponsor of ratification in the Maryland General Assembly." But he chided Mathias for "not debating on the floor [at the Republican convention] if you feel that strongly about it, instead of being in a parade outside, holding a sign." (Mathias was pictured at the head of a parade of ERA supporters including such activists as Bella Abzug and Gloria Steinem.)

Mathias jumped from his seat and told Conroy that "the hall was empty" at the time of the parade, and then, as if lecturing his opponent on the art of politics, added: "In this business, you better learn where the action is and then get in front of it."

Mathias also took issue with his party's stand on the selection of judges, a platform plank that was specifically sought by Reagan. "It's a mistake to imply that judges should be chosen on personal and philospohical bias instead of legal ability. That's very shortsighted," said Mathias. "As a matter of fact, at all of the confirmation hearings on which I have sat as a member of the Judiciary Committee, it has been the absence of bias that we have looked for."

Despite the organized opposition of antiabortion and far-right groups, and the massive Democratic registration edge, Mathias is likely to have little trouble disposing of the Conroy challenge. At a Carroll County Chamber of Commerce luncheon in Westminister, Charles D. Schaffer, an elderly man who is a longtime Mathias booster, may have summed up Conroy's dilemma when he criticized Mathias for his liberal voting record, but concluded with a question that amounted to an endorsement.

"Senate Mac," said Schaffer, "let's forget the past [and his voting record] and get to work solving the problems of this great country. The middle class is being so eroded, Senator Mac, that we'll soon have you in it. So stop using that left oar so much and start pulling to the right, and get the ship of state back on the course."