OLYMPIA — Attorney General Rob McKenna is preparing to officially launch his gubernatorial campaign, hoping to become Washington’s first GOP governor in more than a quarter of a century, two Republicans familiar with his planning said Tuesday.McKenna will formally announce his 2012 bid at a 6 p.m. event today at Sammamish High School in Bellevue, according to the two Republicans who spoke on condition of anonymity so as not to pre-empt McKenna. His political advisers would only say that he planned a “major announcement.” Washington hasn’t had a Republican governor since John Spellman left office in 1985. A year before that, McKenna began his political career as student president at the University of Washington.McKenna, 48, was first elected attorney general in 2004 after serving several years on the King County council. He is also president-elect of the National Association of Attorneys General.During his time as attorney general, he has focused resources on fighting methamphetamine abuse, sexual predators and identity theft. Perhaps his most controversial move came last year, when he angered Democrats such as Gov. Chris Gregoire by signing on to a multistate challenge of President Barack Obama’s health care overhaul.