President Carter, deriding what he called Ronald Reagan's "secret plan" to free the American hostages in Iran, swaggered through conservative central and east Texas today with a hawkish message about American military power near the Persian Gulf.

Addressing an airport rally here, Carter also proudly sported a pair of boots given him earlier today in Beaumont, Tex., and told the crowd the boots would come in handy because "Republicans have a habit of spreading a lot of horse manure around right before an election."

"And lately," he added, "it's getting pretty deep all over the country."

For six weeks, the president has campaigned as the candidate of "peace," expressing his commitment to nuclear arms control and suggesting that Reagan's election would increase the risk of war.

But today, campaigning in this critical, traditionally military-minded state where he trails his Republican opponent, Carter spoke not of arms control but of military might and his willingness to use it if necessary.

He told several thousand people at the rally that the United States now has "the most powerful naval force ever assembled in the Indian Ocean" and enjoys "overwhelming naval and air superiority" in the Persian Gulf region.

"I can tell you, ladies and gentlemen in Texas, we're ready," he said.

In the last several days, Reagan has twice raised the explosive hostage issue. On Tuesday, the GOP nominee called the Americans' long captivity a "disgrace" and "humiliation" to the country. He said he had "some ideas" on how to win their release but did not think this should be discussed publicly.

Carter, who benefited politically from the hostage crisis during the Democratic primaries, has warned his opponents throughout the year not to jeopardize the hostages' safety by making them a political issue. He took this tack Tuesday while campaigning in Florida, refusing to respond to Reagan's charges and saying the fate of the hostages was too important to become "a political football."

Reagan said today in Shreveport, La., that he was going to stop talking about the hostages. The president, however, was willing to let the subject drop. He raised it himself at the airport rally, quickly transforming Reagan's reference to "some ideas" into his "secret plan."

Adopting the same tone of ridicule he used Tuesday to deride Reagan's intellectual abilities, Carter reminded the rally audience of another "secret plan" of an earlier Republican presidential candidate.

"Do you remember when Richard Nixon said just before an election in 1968 that he had a secret plan to win the war in Vietnam?" he asked. "Do you remember that? Well, here it is 12 years later and we still don't know what Mr. Nixon's secret plan was to win the Vietnam war.

"Now, how many of you Texans, with sound judgment, familiar with history, believe that Ronald Reagan has a secret plan to get the hostages back? Gov. Reagan, so far, has . . . done pretty well in keeping his plan secret."

Carter extended this theme to domestic issues, saying Reagan had "a secret plan for providing for the well-being of retired Americans by calling four times to make Social Security voluntary; a secret plan for helping working families by being against the minimum wage; a secret plan to take care of working families who are temporarily unemployed without unemployment compensation."

In the past Reagan has spoken critically of the minimum wage and unemployment compensation, but he does not advocate doing away with either and he has not suggested a voluntary Social Security system since the 1960s.