You can get a better idea of what politicians would like voters to believe from the political ads they authorize than from what they say in speeches or congressional testimony. After all, most speeches will at best be covered in one evening's newscast or morning's newspaper; but a television commercial, as everyone who has heard of ring- around-the-collar knows, can be repeated over and over. So we have watched with some fascination the TV advertisements the Republican Party has aired, at the cost of more than $1 million, this season.

The first advertisement, featuring actors who resemble Speaker O'Neill and former president Carter, has been withdrawn and is generally conceded to have been a dud. The problem may have been that the situation portrayed--the reading of a will--is one that is more familiar and more a focus of great anticipation and great foreboding for a small class of wealthy Americans than it is for the great mass of voters. And the substantive message of the ad--that the Democrats bequeathed inflation and recession to the Reagan administration--is one that does less to advance a positive Republican message than it does to provide an alibi for what voters may regard as Republicans' failure to fulfill every campaign promise.

A more recent advertisement, running from July 6 to 15 in 58 media markets, shows a folksy letter carrier delivering Social Security checks "with the 7.4 percent cost-of-living raise that President Reagan promised." The Democrats are, understandably, squawking: the raise results from legislation passed long before Mr. Reagan took office, and proposals to delay it were made by Reagan administration officials and put aside only when it became clear they were unpopular with the voters. But this one ad seems unlikely to change the public impression, which has remained steady for nearly 50 years, that the Democrats are more inclined to favor Social Security increases and less inclined to favor cuts than the Republicans.

What the ad does show is that the president's party is busy trying tos, unless cut losses rather than make gains. Perhaps that is the best the Republicans can do at a time when unemployment is exceedingly high and when the general feeling is that the president's economic programs are bringing inflation down only at a cost far higher than advertised. But this kind of ad will do little to change the negative tone of current politics--particularly since it suggests limitless devotion to benefits that sensible people in both parties will concede, at least privately, are going to have to be brought under much tighter control.