EVERY TIME we hear how President Reagan is going to hit the campaign trail to produce votes for Republicans, we think of the time President Johnson campaigned in Michigan back in 1966. After a speech enumerating his administration's successes to a listless crowd, Mr. Johnson went on to endorse the Democratic Party's state ticket that year. The only problem was that he mispronounced the name of the candidate for governor, Zolton Ferency, in a way that made it clear that he had never before set eyes on that particular combination of letters until he saw it in his speech text. Mr. Ferency, a man of some humor, gave thanks for his endorsement from "LBQ." But the damage was done, and, despite the presidential appearance, practically every Democrat on that platform lost that year.

The incident illustrates some of the limits on the effectiveness of presidential campaigning. People suppose that a president can saddle up Air Force One, ride into a state and save a flagging candidacy from disaster. The reality is a little different. Voters are not inclined to accept presidential advice in gubernatorial elections, for example, because they are serene in the confidence that they know more about state issues and the personalities of the candidates than the president does. Presidents, after all, have more important things to do with their time than keep up on legislation in Albany or the antics of candidates in Chicago. In elections for the House of Representatives, voters may know as much about incumbents as the president does, and even in senatorial races, the president -- any president -- is not going to swing many votes.

No wonder that President Reagan's advisers are divided on what he should do in the fall campaign. What the president can do is what this president has been pretty successful at all along: framing the issues, establishing a public agenda, setting a tone for political debate. Mr. Reagan will do this every time he appears on the evening news, whether he is televised shouting a few words while entering a helicopter or delivering a peroration at a political banquet. He will dominate discussion of public affairs most effectively if he is seen to be handling well a genuine international crisis -- as President Kennedy did in the mid-term campaign of 1962. He will hurt his party's fortunes most if he is seen to be petty and divisive -- as Richard Nixon seemed in that scratchy television film of a Phoenix speech just before the election of 1970.

In an important sense, the election will be a verdict on Mr. Reagan's policies and approach to government. But unless all the polls and signs are wrong, the response of the voters is likely to be the Scotch verdict of not proven: they are dissatisfied with the results of his program so far but they believe there has not been enough time for a fair test. Mr. Reagan's ability to affect the outcome in any major way by what he does between now and Nov. 2 -- short of a national emergency -- seems limited. To watch his campaign schedule with interest is not necessarily to think it will directly determine the outcome of many campaigns.