President Reagan returned to the campaign trail with a vengeance last week, denouncing "liberals in Washington" with a fire and irrelevance reminiscent of his days on the conservative banquet circuit a generation ago.

There is something simultaneously wonderful and terrifying about a president who regards the government he has led for the past 5 1/2 years as an alien invader from another planet. Only Reagan would have the audacity and the amnesia to berate the congressional "big spenders" after a presidency that has helped quadruple the federal deficit and double the national debt.

Nonetheless, the president's political trip to Texas, Florida and South Carolina was a learning experience for those of us who have watched Reagan campaign for the past two decades.

For starters, we learned that the president needs some new speech writers or at least some new speeches. His long-winded ramblings in Dixie were so devoid of quotable rhetoric or substance that they failed to qualify as stories for the evening network newscasts. Except when Reagan was denouncing Cuba and Nicaragua for a Cuban-American audience in Miami, his speeches also largely failed to stir even partisan audiences.

Perhaps the surviving White House speech writers are playing it safe these days with an eye to their continued employment. Two of the most gifted members of the White House speech-writing team, Peggy Noonan and Ben Elliott, departed after internal conflicts in which they were criticized by White House chief of staff Donald T. Regan and others for supposedly "making policy" in their speech drafts. The authors of last week's forgettable diatribes should be spared such accusations.

Reagan, with a performer's feel for a crowd, seemed to sense that something was wrong and ad-libbed frequently in an effort to rouse his audiences. But the president can be in dangerous airspace when he flies away from his text. In Miami, he swooped down to the barnyard and compared the Democratic platform to a pile of manure. Elated by Senate confirmation of Daniel A. Manion, the president then offhandedly described opponents of this underqualified federal judicial nomination as "a lynch mob."

We were also reminded during this campaign swing that the phrase "Let Reagan Be Reagan" is the most redundant political slogan of all time. There is no way to stop Reagan from being himself, which makes him both the joy and the despair of his advisers.

Here is the Great Communicator, kowtowing to the backward government of P.W. Botha and referring to the nation he misrules not as South Africa but "South America." Here is Reagan two days later in South Carolina opening the door to the sanctions he had opposed while his spokesmen blandly insisted he had been misunderstood.

And here is Reagan in Texas, blaming Democrats for everything except the drought and identifying with the popular distrust of politicians he has done so much to engender. Typically, Reagan quotes "somebody" as saying, "98 percent of the adults in this country are decent, hard working Americans. It's the other lousy 2 percent who get all the publicity, but we elected them."

We also learned last week that Reagan retains what former senator Howard H. Baker Jr. (R-Tenn.) once called "the capacity to surprise." The latest surprise was an orchestrated boomlet by his political strategists for a third term, complete with partisan chants of "four more years."

As is often the case with Reagan, things are not quite what they seem. While the cries for a Reagan third term must send shivers down the spine of Vice President Bush and other pretenders to the throne, the White House is not about to launch a campaign for repeal of the 22nd Amendment. The president is on record as favoring such repeal only for future presidents, not for himself.

What Reagan did last week was remind those who are gazing prematurely into the future that he remains the nation's dominant political figure and that he has no intention of becoming a caretaker president. In continuing to present himself as an antidote to the political system, he has proved the most effective politician of our time.

Reaganism of the Week: Speaking to a fund-raising reception in Miami last Wednesday, the president said, "When I first became a Republican and started attending some Republican meetings some years ago, I would go home to Nancy and say, 'You know, the only young people there looked like they couldn't join anything else.' "