John Smith, who succeeds Neil Kinnock as Labor Party leader on July 18, could determine the fate of the Maastricht treaty on European unification. Parliament is expected to vote on the treaty in the fall. If Labor joins forces with anti-Maastricht Conservatives, the treaty could be defeated. Smith has been known for taking pro-Europe stances, but Labor politicians, who see a chance to strike a blow at Prime Minister John Major's government, are pressuring him to switch. A defeat in a key country such as Britain could force Maastricht back to the drawing boards.