Republican gubernatorial candidate Jeff Johnson on Sunday proclaimed he'll be a champion of Minnesota's middle class, but stumbled when asked how to define them.

During Sunday's Fox 9 debate at Hamline University, Johnson said "I have no clue how I would define that."

The remark was immediately seized on by Gov. Mark Dayton's campaign and the DFL, which put out an ad deriding his opponent. But, even the governor had a tough time defining what middle class is, offering his best guess of a total annual household income of between $50,000 to $60,000.

Turns out, both might be right.

Economists, sociologists and political scientists have not decisively defined what it means to be middle class. Disparities in cost of living in different regions and cities also makes it difficult to pinpoint income brackets for this subset of Americans. Moreover, public opinion polls find views on what it means to be middle-class vary widely.

A 2013 Wall Street Journal/NBC News Poll found that nine in 10 Americans believe the top threshold for middle class are families with a total income of $100,000. The tendency, according to this poll, is Americans more often than not believe their own income brackets to be considered middle class.

Dayton's guess appeared more in line with the majority of Americans in this particular poll. Half of those with total annual incomes of $50,000 to $75,000 considered themselves to be middle class. That's compared to 29 percent of all adults who believed the $50,000-$75,000 bracket to be middle class.

A 2013 story in the Cincinnati Enquirer found various estimates by credible groups on how to define middle class based on income levels. Unsurprisingly, the brackets varied widely.

"In the past few years, the "middle class" income range has been described as between $32,900 and $64,000 a year (a Pew Charitable Trusts study), between $50,800 and $122,000 (a U.S. Department of Commerce study), and between $20,600 and $102,000 (the U.S. Census Bureau's middle 60% of incomes)," Dan Horn at the Enquirer wrote.

State Democrats are seizing on Johnson's "I have no clue" remark, saying it undercuts his argument that he'll be an advocate for the "forgotten middle class" has he said during Sunday's opening statements.

"Today's debate made crystal clear that Jeff Johnson is completely clueless about how to strengthen middle class families," said DFL Chairman Ken Martin in a statement.

Jeff Bakken, a spokesman for the Johnson campaign, said that it's ironic Dayton, the great-grandson of the founder of Dayton's, a department store chain that spawned Target Corp, is questioning Johnson's commitment to the middle class. Johnson grew up in Detroit Lakes in a middle-class household, Bakken said.

"Jeff Johnson was born and raised in Detroit Lakes, his dad delivered bread to supermarkets for a living, and Jeff has earned every dollar he's made," Bakken said in a statement. "If Mark Dayton and his attack machine want to get into a debate with Jeff Johnson over who better understands the middle class, bring it on."