IT MAY BE a dying art form, but a seasoned performer can breathe life into it. The art form is the presidential campaign trip, and the seasoned performer is Ronald Reagan. To audiences in the Dallas Apparel Mart, Miami's Intercontinental Hotel and the University of South Carolina Coliseum, the president denounced "big taxers and big spenders" and "liberals in Washington" and praised the nation's economic "expansion" -- "call it confidence, self-assurance, what you will."

It's easy to find things to criticize in the president's presentation. The "expansion" he cited has proved disappointingly small, and he signed bills authorizing and appropriating almost every penny of the "big spending" he criticized. After six years in the White House, he is still talking as if "liberals" were in control of the government in faraway Washington and he were still a citizen-broadcaster from California.

Do these messages help Mr. Reagan? He does get a minute and some seconds on the network newscasts to deliver his remarks. But he's not running for anything (despite the ludicrous effort of the House Republican campaign committee to bamboozle potential contributors into thinking they're trying to change the Constitution so that Mr. Reagan could seek a third term in 1988). And he's able to get on the tube pretty much when he wants to. Voters can easily see that he's just restating general themes he has sounded since 1966; they realize that he's glossing over such embarrassments as the deficit.

So is Mr. Reagan helping the candidates he's stumping for? He can certainly help them raise money. But how many votes does he swing for gubernatorial candidates Bill Clements in Texas and Carroll Campbell in South Carolina or for Sen. Paula Hawkins in Florida? The president inserts phrases in each speech referring to the candidates and dovetailing with the themes of his TV spots. But voters probably know more about state issues and the character of local candidates than Mr. Reagan does. This, in other words, is mainly traditional hoopla, a perennial: the president on the hustings. True, there are some pretty tight races involved, and true, the presidential forays could have some effect, but as well as the current incumbent performs the role, it remains a politically much overblown event.