Call it March of the Democrats. Thanks to his environmental doc, An Inconvenient Truth, Al Gore has joined penguins, potty-mouthed comedians, and pubescent spellers on the list of unlikely recent nonfiction stars. After three weeks in limited release, Truth has already earned just under $4 million. And this weekend, Paramount Classics will expand the film to 400 theaters nationwide, well into uh-oh! red-state territory. ''The big question is suburbia,'' says Paramount Classics president John Lesher, who bought the film at Sundance. ''Will it reach the critical mass there?'' If its strong performance continues, Truth should surpass Bowling for Columbine's $21.6 million tally to become the third-highest-grossing non-IMAX, non-concert doc ever (behind Fahrenheit 9/11 and March of the Penguins). But even though Truth has earned more than Penguins did after three weeks, Lesher admits topping its $77.4 million gross may be impossible: ''You got kids and adults into that.'' Not to mention Republicans.