Edmund S. Muskie, billing himself as "the first political secretary of state in a long time," waded into battle against the Republican opposition today while dealing cautiously with continuing speculation that he himself might head the Democratic presidential ticket.

The veteran national politician, in the first of a series of planned speaking tours, attacked "a foreign policy of reaction" in the hands of "fear merchants" who would bring about "a military buying binge" and other ills in years to come.

Muskie did not identify his target by name, but left no doubt in the minds of his audience at the United Steelworkers of America convention that he was speaking of the Republican Party and its presidential nominee, Ronald Reagan.

In a prepared speech tonight for the American G.I. Forum, a Hispanic group Muskie struck at "partisan doubletalk" which asks "a blank check for massive military spending" at the time as "a massive tax cut."

The secretary of state dwelt at length with the benefits and importance of the Panama Canal treaties, which were opposed by Reagan to the end. In an apparent reference to the GOP nominee, Muskie said "some might even try to reverse this step [the treaties] if they were given half a chance."

Perhaps because he spoke so much of military matters, defending both the Carter administration's heavy arms spending and its backing for arms control, Muskie in a slip of the tongue identified himself to the steelworkers as "secretary of defense."

When he caught the slip and correctly named his Cabinet post, Muskie said he intends "to play that job just like the other political jobs I've known."

The former Maine senator and governor gave no encouragement to those who have suggested he be the nominee if President Carter's support collapses at next week's Democratic National Convention. The secretary of state said he is following a maxim he attributed to former first lady Eleanor Roosevelt: "Always be on time, do as little talking as humanly possible, and remember to lean back in the parade car so everybody can see the president."

Heeding this advice, at least on the final point, Muskie added extemporaneous references to Carter at two places in his steelworkers address. He also paid tribute to Carter's leadership during a press conference and said, "I don't foresee any" circumstance that would change his own political stance.

Still, Muskie did not rule himself out as the party's choice if Carter fails of nomination. He refused again to state his position on the hot convention-rules fight that is expected to be the first major test of Carter's strength.

There was no indication of support for a Muskie candidacy among the 3,700 steelworkers here. About an hour after the secretary completed his speech, the convention by voice vote endorsed Carter for the presidency, backing up a recommendation of its executive board. There was no mention of Muskie in the floor debate on the issue.

Responding to reporters' questions, Muskie dealt gingerly with the possibility of an early break in the nine-month captivity of U.S. hostages in Iran. "I wouldn't rule out anything, nor would I raise expectations," he said.

Among other potentially favorable aspects Muskie mentioned the Islamic holy period of Ramadan, which ends next week, but he did not suggest that the holy days are likely to see the captives' release.