Sen. Russ Feingold (D-Wis.) continues to hammer on comments his Republican opponent made about drilling for oil "where it is."

Feingold has used businessman Ron Johnson's (R) remark to suggest if he'd back allowing oil exploration in the Great Lakes.

"[Ron] Johnson said clearly, when he was asked would you support drilling in the Great Lakes, he said I'll drill wherever there is oil," Feingold told the Wisconsin Radio Network Radio Sunday.

A spokeswoman for Johnson, who's emerged as the favorite to win the GOP nod to challenge Feingold, called the suggestion "completely untrue."

Asked if he would support expanding oil drilling in the United States in a recent interview with, Johnson said: "I think we have ... to get the oil where it is but we need to do it responsibly."

Feingold has already used the claim in a TV ad, which prompted a quick rebuke from Johnson's camp.

The incumbent is clearly worried about his reelection prospects. While a recent poll had the senator with a 55 percent approval rating, a recent Rasmussen survey showed him virtually tied with Johnson.

Drilling for oil is currently illegal in the Great Lakes.

"The fact is, this is going to happen again, if people like Mr. Johnson are running the show," Feingold said Sunday. "He said the reason that this happened is that the oil companies were over regulated, not under regulated. Now if anyone else in their right mind believes that, I'd like to understand why. That's the difference between me and Mr. Johnson. He wants weak laws for the oil companies, and want to make sure we don’t drill in the Great Lakes."

Johnson's spokeswoman said the Democrat was "grasping at straws."

--Updated at 7:16 p.m.