Even before Mac Mathias arrived in town, the rumors were around that the liberal U.S. senator from Maryland wouldn't last out the week at this conservative lovefest. The speculation centered on what day his bags and retreat to Washington -- would it be before the anti-ERA platform was officially adopted or before Ronald Regan was nominated.

Mathias today squelched the gossip by announcing he would be here all week. But in a sense, based on what he said and did on this first day of the Republican National Convention, he isn't really in Detroit at all.

When the band struck up its march to mark the opening of the convention, Mathias was nowhere to be found. He skipped the opening ceremonies entirely, choosing instead to join the morning-long march by adherents of the Equal Rights amendment.

Asked if his stand for the ERA went against the grain of a party that had just abandoned this measure, Maithias avoided the chance to score rhetorical points with liberal voters. He merely replied mildly that "the new platform says everyone can exercise his own conscience on this."

When reporters crowded around him at the delegation's morning meeting, asking his opinion of the party's platform, he just smiled and said, "I haven't read it."

When old party friends greeted him in the crowded streets near the convention hall, he answered all questions about Reagan with a hearty, "Great to see ya."

For Mathias, trying to win reelection to a third term in a heavily Democratic state, all this artful dodging reflected pragmatism as much as principle.

He owes his Republican colleagues a loyalty that goes beyond lip service this year. When the Eastern Shore's home-grown conservative, Rep. Robert Bauman, was considering a primary challenge to Mathias, national party officials sent an emissary to warn him against such a move. Mathias was an incumbent, the messenger said. No matter that ideological differences of the past, the party was going to stand behind all incumbents.

At the same time, Mathias cannot stray too openly from the commitment to liberal social policies that have made him a favorite with labor leaders and moderate Democrats throughout his state -- a state where Democrats outnumber Republicans nearly three to one.

So far, his response to the dilemma has been to keep a low profile. When he arrived on the convention floor with his delegation for the evening session, Mathias passed the hours applauding everything with the same faint, polite taps of his hands and responding to almost every potentially troublesome question with the same pleasant vagueness.

"Mac has to walk a very thin line within his own party," explained Tom Bradley, head of the Baltimore Metropolitan Council of the AFL-CIO.But, Bradley added, if Mathias acts too much like a Republican, Democrats may well desert him in November.

Already, Bradely said, some influential Democrats who supported Mathias in the past have voiced dismay at the senator's decision to stand side by side with Bauman as cochairmen of the Maryland Republican delegation.

"The less controversy he stirs up out there, the less he is noticed, the better," Bradley said.

This was no idle warning. Bradley and a dozen other state labor leaders gave Mathias essentially the same message at a picnic on the senator's Frederick County farm a month ago. Mathias can't afford to ignore the advice of men whose organizations gave him $50,000 in campaign contributions during the primary election alone.

But one state party official points out that Mathias is also counting on substantial financial support from the Republican Senatorial Committee, a party fund-raising operation that is dominated by the same kind of conservatives who are controlling the convention.

So he can't afford to offend conservatives. He can't afford to offend liberals. What can he do?

"I do what I do," said the senator, as he headed for a small lunch in honor of George Bush. "I don't feel any pressure to do otherwise."

In practice, aside from marching for the ERA, that has meant doing very little. And saying less.