A lawsuit filed yesterday accuses GOP presidential front-runner George W. Bush of trampling the free-speech rights of environmentalists who protested in front of the Texas governor’s mansion. The enviros, who were speaking out against the state’s air-pollution policies, were arrested by state police four times between March and May of this year, and all four times charges were dropped. Bush spokespeople said the protestors failed to stay within the area designated for demonstrations, and state troopers were simply protecting pedestrians. “The Texas constitution does not stop at the sidewalk in front of the governor’s mansion,” responds Rick Abraham, executive director of Texans United, one of the groups filing suit.