THE REPUBLICAN convention, it turns out, was not quite the boffo box office its backers hoped for. True, a lot of Americans always find something to do in August other than watch television. But even so, the convention of the party in power, traditionally held in August, usually attracts a larger audience than the challenger party's convention. This time that wasn't so. The final night of the Republican convention, featuring an incumbent president who is also a seasoned and attractive TV performer, was watched in 19.1 million American households. The final night of the Democratic convention, featuring a former vice president much less known and generally considered less telegenic, was watched by 19.4 million households.

That doesn't mean Mr. Mondale is ahead of Mr. Reagan. An even bigger audience, the ratings people say, tuned in to watch Jesse Jackson on the Tuesday night of the Democrats' convention, even though Mr. Jackson, in pairings against Mr. Reagan, was getting on the order of 21 percent of the vote. No one doubts that the Republicans' Tuesday night feature speaker, Gerald Ford, could get a lot more votes than Mr. Jackson even though he attracted fewer viewers. Evidently many people watched out of curiosity or in the expectation of seeing a good, if not to them persuasive, show.

Strong backers of each candidate made up much of the audience for each convention. NBC was the only network to run the 18-minute Reagan movie, after much controversy, and apparently enough Reagan fans switched from other networks to make NBC, for that night, No. 1 in the ratings. ABC was No. 1 for the Republican convention overall, and CBS was No. 1 with the larger audience that watched the Democrats in July.

But to put all these figures into perspective, remember that there are 83.8 million "television households," more thathree-quarters of which were not tuned to the conventions at any given time. The real box office winner on the tube this summer was the Olympics. The messages sent out by the parties' conventions are reverberating and reaching voters who didn't tune in. But the politicians are uncomfortably aware that they're trying to persuade an audience that doesn't want to watch them and will go to some trouble -- by watching reruns of reruns -- to avoid doing so.