RICHMOND — Former Virginia Gov. Tim Kaine on Tuesday officially jumped into the 2012 U.S. Senate race in Virginia, setting up a potential marquee matchup with former Gov. and Sen. George Allen in a state vitally important to President Obama’s re-election chances. In an online announcement video, Kaine touted his experience as a mayor and governor in Virginia without mentioning his ties to Obama or his tenure as chairman of the Democratic National Committee, a signal that the Democrat hopes to keep the race focused on the needs of the state rather than the national issues now dominating — and dividing — official Washington.