President George W. Bush gave a boost to Connecticut Sen. Joe Lieberman's re-election bid as an independent by taking the rare step of refusing to endorse the Republican candidate It is highly unusual for a sitting president not to endorse a Senate candidate from his own party and suggests the depth of the political machinations over Lieberman's Senate seat after his defeat in last week's Democratic primary by Ned Lamont, a well financed challenger running on an anti-Iraq war platform. Lieberman, the 2000 Democratic vice presidential nominee, has vowed to launch an independent bid to retain his seat.