The political campaigns will keep things toasty right into November. But if you have an aversion to hot air, you can cool those sinuses at the American History Museum this Saturday. The museum's presentation of "And If Elected. . ." begins at 1 with a guided tour through the memorabilia of past campaigns. At 1:30 Mark Twain impersonator William McLinn performs "Smitten With Presidential Madness," followed at 2:30 by a concert of campaign songs from the 1800s to the present. Both performances are in the museum's Hall of Musical Instruments. The museum is at 14th and Constitution NW. If you're taking Metrorail, exit from the Blue or Orange lines at the Smithsonian station. Call 357-2700; free. THE GIFT OF GOSPEL If gospel music makes you shout for joy, a colloquium on the subject may be music to your ears. The Rev. William Brewster and other gospel eperts will sing and discuss the role of gospel music at the American History Museum's Hall of Musical Instruments. Hours are 2 to 4 on Friday. The museum is at 14th and Constitution NW. If you're taking Metrorail, exit from the Blue or Orange lines at the Smithsonian station. 357-4176; free.