ABC News' Jonathan Greenberger Reports: Republican presidential candididate Mitt Romney offered a new explanation today for why he supported a Democrat in 1992. That year, Romney, then a registered independent, voted for former Sen. Paul Tsongas in the 1992 Democratic presidential primary. He told ABC's George Stephanopoulos, in an interview that will air Sunday on "This Week," that his vote was meant as a tactical maneuver aimed at finding the weakest opponent for incumbent President George H.W. Bush. "In Massachusetts, if you register as an independent, you can vote in either the Republican or Democratic primary," said Romney, who until...