The bombshell announcement by Pennsylvania Sen. Arlen Specter that he will leave the Republican Party and seek re-election as a Democrat dramatically reshapes the 2010 Senate race in his state.

In shedding his longtime party label, Specter has decided to avoid a potentially disastrous rematch in the Republican primary against former Rep. Patrick J. Toomey, a conservative who nearly defeated Specter in 2004 and who is leading the senator in early polls.

As a result of Specter’s decision to run as a Democrat, CQ Politics is changing its rating of the Pennsylvania Senate race to “Leans Democratic” from the tossup category, “No Clear Favorite.”