WASHINGTON — Tag. I’m not it. Republicans considered to be up-and-comers are scrambling to declare a lack of interest in becoming Mitt Romney’s running mate, taking themselves off the still-forming short list of would-be vice presidents. With Romney poised to win the GOP nomination in June, if not earlier, some of the focus has shifted to his pick for the No. 2 spot on the ticket. But no one is rushing forward and many of the top prospects are trying to shut down the conversation before it begins.