(CNN) - Democratic Sen. Arlen Specter of Pennsylvania is deadlocked with probable Republican opponent Pat Toomey in his difficult bid for re-election this year, according to a new poll.
A Franklin and Marshall College survey released Wednesday indicates that Toomey leads Specter 33 percent to 29 percent among registered Pennsylvania voters; 32 percent are undecided – a very high number. Toomey's 4 point advantage is within the poll's sampling error. In Franklin and Marshall's last poll, conducted in February, Toomey trailed Specter by 4 points.
Toomey is a former congressman and former head of the Club for Growth, a limited-government and anti-tax organization. Specter, a five-term senator, switched parties from Republican to Democrat last spring. At the time of the party flip, he cited the difficulty in winning the Republican primary against Toomey as a factor.
A Quinnipiac University survey of Pennsylvania voters conducted late last month indicated that Specter led Toomey 49 to 42 percent in a hypothetical general election matchup. That was up from a 44 percent tie in a Quinnipiac poll conducted in December.
The Franklin and Marshall poll also indicates that Toomey leads Rep. Joe Sestak 27 to 19 percent in a hypothetical general election matchup, with nearly half of those questioned undecided. Sestak, a two-term congressman from southeastern Pennsylvania and former Navy admiral, is challenging Specter in May's Democratic primary.
According to the survey, Specter leads Sestak 32 percent to 12 percent in the Democratic primary, but more than half of Democratic primary voters remain undecided. Specter led Sestak 53 percent to 29 percent in the February Quinnipiac poll.
The high number of undecided respondents is due to the fact that the poll explicitly offered respondents an undecided option, which most polls do not do.
"Polls that have the interviewer read an undecided option along with the names of the candidates tend to vastly overstate the true number of undecided voters," said CNN Polling Director Keating Holland. "That's because some respondents know how they will vote but feel impolite or uncomfortable expressing that view, so they jump at the chance to give an 'undecided' answer if that option is offered, even though it doesn't reflect their true opinion. On occasion, a high undecided number can mask one candidate's strength."
Three in 10 people questioned in the Franklin and Marshall poll approve of the job Specter's doing as senator, unchanged from last month. And 28 percent say Specter deserves re-election, up 3 points from February.
The survey indicates that Pennsylvanians say health care is the most important issue in their vote for Senate, followed by the economy, a switch from February when the economy was on top.
The Franklin and Marshall College poll was conducted March 15-21, with 964 registered Pennsylvania voters questioned by telephone. The survey's sampling error is plus or minus 3.2 percent for registered voters and plus or minus 4.5 percent for questions only of Democratic respondents.
Follow Paul Steinhauser on Twitter: @psteinhausercnn