WHAT ARE presidential candidates saying about AIDS? They tend to agree on many things: the need for aggressive research, for compassion and care for AIDS victims, for steps to stop the spread of the disease, for public education. But on some subsidiary questions they, like other Americans, disagree.

The Democrats tend to attack mandatory testing as unnecessary and ineffective. Jesse Jackson and Michael Dukakis oppose mandatory testing of anyone, even prisoners and immigrants. Richard Gephardt and Bruce Babbitt speak out against across-the-board mandatory testing, but leave open the possibility of testing some groups. Paul Simon, Albert Gore and Joseph Biden voted in the Senate for mandatory testing of immigrants. Insurance issues also split some Democrats.Michael Dukakis ruled that life insurance companies (but not health insurers) in Massachusetts can require AIDS tests, which prompted his insurance commissioner to resign. But it's hard to see why companies that can require every other test can't require this one.

For the Republicans, the debate seems to be over which part of Surgeon General C. Everett Koop's statements on AIDS you emphasize. Dr. Koop said the best defenses against AIDS are chastity and monogamy but that early education of children and widespread use of condoms are also needed. Candidates of both parties sensibly repeat the first bit of advice, but the second raises hackles. In June Jack Kemp and Bob Dole withdrew their sponsorship of a dinner for Dr. Koop -- a bit of demagoguery, particularly for Mr. Dole, who spoke out in the Senate against an overbroad mandatory-testing provision. George Bush and Jack Kemp favor testing for prisoners, immigrants and -- a more numerous category -- marriage license applicants. So does Alexander Haig, who calls AIDS "a moral challenge" as well as a medical problem. Pete du Pont, who says AIDS is "a health issue with moral implications," agrees with tests for the first two groups, but is unsure about the marriage license applicants. Some Democrats, notably Bruce Babbitt, denounce the idea of testing marriage applicants. But this is one question to which "not sure" may be the prudent response until more is known about the spread of the disease.

There has been less demagoguery on the AIDS issue than many feared, but there have been occasional bursts, such as Pat Robertson's suggestion that state or local governments might have to "apply some sort of quarantine of AIDS victims." There have been bursts of hyperbole as well, such as Bruce Babbitt's saying the Reagan administration has "a voodoo health policy." On AIDS there is more agreement than disagreement among the candidates -- and a feeling that much more needs to be known about the disease.