The Senate confirmed Scott Pruitt on Friday to run the Environmental Protection Agency , putting a seasoned legal opponent of the agency at the helm of President Trump’s efforts to dismantle major regulations on climate change and clean water — and to cut the size and authority of the government’s environmental enforcer. Senators voted 52 to 46 to confirm Mr. Pruitt, the Oklahoma attorney general who has built a career out of suing to block the E.P.A.’s major environmental rules and has called for the dissolution of much of the agency’s authority. One Republican, Susan Collins of Maine, crossed party lines to vote against Mr. Pruitt, while two Democrats, Joe Manchin III of West Virginia and Heidi Heitkamp of North Dakota, both from coal-rich states where voters generally oppose environmental rules, voted for him.