WASHINGTON — Battling brain cancer, John McCain on Thursday vowed to return to the Senate, leveling fresh criticism at the Trump administration and aiming a good-natured dig at Republican and Democratic colleagues shaken by news of his diagnosis."I greatly appreciate the outpouring of support — unfortunately for my sparring partners in Congress, I'll be back soon, so stand-by!" McCain said in a tweet. Showing no signs of stepping back from political and national security battles, he issued a statement slamming the Trump administration over its Syria policy.The 80-year-old McCain, the GOP's presidential nominee in 2008 and a six-term Arizona lawmaker, was diagnosed with glioblastoma, an aggressive type of brain cancer, according to doctors at the Mayo Clinic in Phoenix, who had removed a blood clot above his left eye last Friday.