It's the kind of sneaky grassroots marketing that would make the Blair Witch guys smile. Since February, Twentieth Century Fox has been waging an extensive campaign including a website, a TV spot (airing late at night, mostly in New York City, L.A., and Washington, D.C.), bumper stickers, and buttons to rally the populace behind Sen. Robert Kelly and his anti-mutant legislation. The catch? Kelly is the fictional right-winger trying to thwart the mutant superheroes in July's X-Men movie. ''We got the ball rolling,'' says Jeffrey Godsick, a top Fox publicist, ''and fans have decided to give it a life of its own.'' Indeed, X-Meners toting pro-Kelly placards recently showed up at the Today show. More pranks are planned May 30 in Chicago outside the Daley Center municipal building and at a Pennsylvania Reform Party meeting in June. What's next, Mutant Day at Wrigley Field?