Rep. Mark Kirk (R-IL) will not run for the open seat of Sen. Roland Burris (D-IL) in 2010, the Washington Post reports.
It's a "stunning reversal from just 48 hours ago" when Kirk signaled that he would be a candidate.
Kirk's decision -- "a blow to Senate Republicans' chances in Illinois" -- came in the wake of Burris' formal retirement this afternoon.
Update: Roll Call says Kirk is still mulling the race and that he is trying to avoid a primary with state Republican Party Chairman Andy McKenna. Kirk and McKenna are discussing the race over the weekend.