Chris Cillizza reports that Murtha’s Pennsylvania 12th District is the only one in the country to have been won in 2004 by John Kerry and in 2008 by John McCain. May 18 is the natural choice for a special election to replace the passed Vietnam veteran and 18-term congressman:

According to state law, the governor has ten days once the vacancy is officially declared to decide on the date for the special election, which can come no sooner than 60 days following that proclamation.
That likely means the special election will be held on May 18, which is the date already set for federal primaries around the state. (Special elections costs the state huge sums of money and it’s likely that Gov. Ed Rendell will choose to go with an already established election day to save some cash.)

UPDATE: Cook Political Reports now rates the race for Pennsylvania’s 12th a toss-up.