State Treasurer Rob McCord had 8 percent; former Auditor General Jack Wagner of Pittsburgh, who declared his interest in the race last week, had 7 percent, as did former environmental secretary John Hanger. Katie McGinty, who also was once the state's chief environmental official, had 6 percent. Nineteen percent of respondents said they were undecided.
McCord has been elected twice statewide, and Schwartz, who represents the 13th District, has a base in the vote-rich Southeast. Neither has advertised yet in the governor's race.
The winner of the May 20 primary will face Republican Gov. Corbett in the fall.
"The real impact in the race has been Wolf's spending," pollster Brock McCleary said. "In fact, it has given form to a race that to this point has been filled with candidates who lack broad name identification beyond niche constituencies."
Wolf's name recognition jumped to 55 percent, up from 23 percent in a November poll.
"It's definitely good news, but there's still a lot of game left," said Wolf campaign spokesman Mark Nicastre.
Wolf has spent about $2.7 million on ads across Pennsylvania, according to Democratic sources monitoring spending. The largely self-funded candidate began the year with $11.1 million on hand.
McGinty began running ads Monday in all the state's TV markets except Philadelphia. Strategists for Wolf's rivals predicted the race would tighten as others follow.
The latest survey did not include Lebanon County Commissioner Jo Ellen Litz or the Rev. Max Myers of the Harrisburg area, who quit the race Monday.
The automated phone poll contacted 501 likely primary voters. Harrisburg-based Harper Polling put the margin of error at plus or minus 4.38 percentage points.