With 31 percent of voters undecided, the special election to fill the seat of the late Rep. John Murtha (D) is shaping up as a tight race, according to a Susquehanna Polling and Research survey.

The poll, conducted March 14-15 for the Pittsburgh Tribune-Review, found Democrat Mark Critz, a former Murtha aide, ahead of Republican businessman Tim Burns, 36 percent to 31 percent.

The survey had a 4.9-point margin of error.

The special election to fill Murtha’s seat for the rest of this year takes place May 18. On the same ballot, voters will choose the nominees who will run for the seat in November.

Looking ahead to November, 59 percent of those surveyed said they were undecided.

Critz led the Democratic field as the favorite of 30 percent. None of the other Democrats — Navy veteran Ryan Bucchianeri, Cambria County Controller Ed Cernic and attorney Ron Mackell — was favored by more than 4 percent of those surveyed. Cernic has since dropped out of the race and endorsed Critz.

Voters also were uncertain about their Republican primary choices, with 37 percent calling themselves undecided, 32 percent preferring 2008 GOP nominee Bill Russell and 30 percent favoring Burns.

The survey’s sample size was 400 likely voters in the May 18 elections.