Marquette Law School's latest survey finds Gov. Walker and Democrat Mary Burke tied at 46 percent, among registered voters.
However, among people who say they're certain to vote in November, Walker leads Burke 48-45 percent.
Poll Director, Prof. Charles Franklin says turnout therefore could be a key determinant. "These results show that the election outcome can change depending on turnout, as both parties seek to mobilize their supporters," Franklin said.
The poll's margin of error is +/- 3.5 percentage points.
Six percent of registered voters responded that they remain undecided about whom they'll vote for in the governor's race. Burke is cited as the party front runner, among four Democratic hopefuls.
In March, the last Marquette poll, the incumbent Republican governor led Burke 48-41 percent among registered voters.
While Walker has lost ground, his approval rating has improved. In March, 47 percent of respondents approved of the job he was doing, while 47 percent disapproved. This month, the governor's approval rating stands at 49 percent, versus a 46 percent disapproval rating.
As for voters' familiarity with Burke, 51 percent say they don't know enough about her to form an opinion. The rate sat at 70 percent in January and 59 percent in March.