Maryland Sen. Charles McC. Mathias assumed a familar role yesterday, waving the ragged banner of Republican liberalism by urging the party to reaffirm its traditional support of the Equal Rights Amendment.

If the Republican platform committee at the Detroit convention fails to carry on the 40-year tradition of support for such an amendment, Mathias said at a Capitol Hill press conference yesterday, "the party of freedom no longer deserves its name."

At Mathias side during the press conference were Reps. S. William Green (R-N.Y.), who originally drafted the pro-ERA letter that was sent to the party platform committee, and four of the 43 other congressional Republicans who joined Mathias and Green's declaration of support for the ERA.

Prospective Republican presidential nominee Ronald Reagan long has opposed the amendment and in a Chicago press conference June 20 called it a "divisive symbol," suggesting instead that the party platform oppose sex discrimination without endorsing the ERA.

The liberals' open break yesterday with Reagan's position produced no hint of dissatisfaction or discontent from the conservative forces who for weeks have been dominating Republican party politics.

"We have a delicious situation, from a conservative viewpoint, of a major liberal figure like Mac (Mathias) having to seek to influence our candidate," chortled Maryland's conservative congressman Robert Bauman.

"I'm flattered, as a conservative, that Mac feels he has to do this," added Bauman. The two men, frequent ideological adversaries in the past, this year are serving as cochairmen of the state party's convention delegation in the name of unity.

Mathias is considered a strong favorite in his reelection bid this year, but finds himself in an awkward position within his own party, which has moved farther to the right of him.

He voted for the Panama Canal treaties. Reagan opposes them. He voted against the Kemp-Roth 30 percent, three-year tax cut. Reagan has favored it.

He voted against the neutrom bomb, which Reagan supports, and he voted for U.S. support for abortions, which Reagan opposes.

Like most liberal Republicans, Mathias has kept quiet in recent weeks, muting the intraparty ideological differences that have left Republicans divided and defeated in the past.

When Republican senators rallied behind a 10 percent tax cut proposal -- a one-year miniversion of the Kemp-Roth bill -- Mathias, like fellow liberals Charles Percy (R-Ill.) and Jacob Javits (R-N.Y.), went along. l

"[mathias] definitely feels there's a necessity for unity," said one aide who asked not to be named. "That [the tax cut] was one particular vote, I think, when he felt he could make that known."

It has all been a very delicate balancing act, but when it comes to the ERA -- an emotional issue but hardly a key factor in Republican party strategy -- Mathias decided to openly break ranks. Rep. Green's staff asked for his help on their letter to the platform committee, and Mathias agreed to pass it around to the Senate's 35 Republicans.

"It would be a step backwards if we abandon it (support for ERA)," Mathias yesterday. "This is well-trodden ground. This is where the Republican party has been for 40 years. If there's movement away from this, thas has significance . . .

"Anytime you take a position you risk polarizing other people," he added. "But this isn't a position I'm taking. This is the position of the Republican party until its changed."

Will he speak out on other potentially divisive issues before the convention begins in two weeks? "I have no particular plans to," Mathias said. "THIS ONE (ERA) just looked like it was getting early attention."