LANCASTER, Pa. -

Pennsylvania's primary election is less than three weeks away, and the race for governor is entering the home stretch.

The Democratic candidates went head to head Thursday night in a debate in Lancaster, sounding off on issues like education, property taxes and natural gas drilling. 

One local political expert said, when it comes to taking down Republican Gov. Tom Corbett, the time to strike is now. 

Chris Borick, director of the Muhlenberg College Institute of Public Opinion, said Democrats are poised to overthrow Corbett in November.

"He has low public approval ratings, and the Democrats have thoughts from the start this is a great opportunity to knock off the incumbent Republican," Borick said of Corbett.

"Tom Corbett last year tried to shut down the defined benefit plan and say, we'll just move to 401k's and everything will be fine," candidate Rob McCord said during the debate. "That's called the great risk shift, so workers don't know what they'll be able to retire on."

First, Borick said, the candidates need to focus on the May 20 primary. 

According to a survey out this week from Muhlenberg, businessman Tom Wolf is leading with about 40 percent of Democratic voters likely to choose him to run against Corbett.

McCord, the state's treasurer, and U.S. Rep. Allyson Schwartz are neck and neck for second place, each with about 15 percent.

During the debate, Schwartz spoke passionately about natural gas drilling. 

"We do also have great opportunities with the energy, not just to drill that natural gas but to look at and work with local universities to see what are the byproducts and how else must we use them," said Schwartz.

Kathleen McGinty trails with two percent.

"What I promise and what I bring is a person who has great vision for Pennsylvania. We can compete and win in advanced manufacturing and in an innovative and entrepreneurial economy," said McGinty.

Borick said debates like Thursday's are crucial for the candidates trailing Wolf, who made his position on education and economic issues known very early in the race.

"I think we have to start by making sure we keep the promises we made to state workers, including teachers," Wolf said during the debate.

"You can see that by the tone of recent debates, recent ads, where the candidates are targeting Tom Wolf to try to bring him back," said Borick.

Corbett's only Republican challenger was tossed off the ballot Thursday.

The state Supreme Court said Bob Guzzardi failed to file his statement of financial interests on time.