Rob Zerban thinks he can knock off a Wisconsin giant next fall.

And Democrats on Capitol Hill agree that this 42-year-old businessman, a former Republican with limited experience in county politics, could be their best shot at defeating House Budget Chairman Paul Ryan (R) in more than a decade.

“Nobody had any illusions about how difficult the race would be,” Zerban said late last week, reflecting on recruitment calls from the Democratic Congressional Campaign Committee and other groups that persuaded him to enter the race last month. “They said: ‘You have to realize it would take a lot of money and would be an uphill battle. But now it’s a good time to try.’”

Indeed, as Ryan’s national profile balloons, his 2012 re-election campaign in Wisconsin’s 1st district could be growing more difficult.

The seven-term Congressman is now the face of the GOP plan to transform Medicare, an issue largely blamed for an embarrassing Republican defeat in western New York’s special election last week. And recent polling suggests that Ryan’s popularity among independents and Democrats, a group he will need to hold his moderate district, is falling.

“The more he’s front and center on Medicare, the better chance I think I have,” Zerban said.

Democrats certainly believe they have an opportunity to nationalize the Wisconsin House race, driving resources and energy from across the country into a swing district Ryan has held relatively comfortably since 1998. And they believe that fallout from New York’s GOP debacle will help.

Zerban created a federal campaign committee roughly six weeks ago but sent his first email fundraising solicitation only the day after the Empire State’s special election.

“After last night’s loss, Paul Ryan’s Republican friends will rally behind him and prepare him for the fight of his life. We must do the same,” Zerban wrote. “It’s time for us to show Paul Ryan that we won’t let Congress break its promise to our seniors.”

Expect liberal groups such as the Progressive Change Campaign Committee, which has already been in touch with Zerban, to use their national networks to help defeat Ryan. And DCCC Chairman Steve Israel (N.Y.) went out of his way last week to prop up Zerban’s candidacy.

“In Paul Ryan’s district, Wisconsin 1, we have Rob Zerban, an outstanding recruit, who decided to come on because he couldn’t stomach the fact that his Member of Congress was the architect of the blueprint, was the architect of a plan that would end Medicare in order to fund tax cuts for big oil companies,” Israel told reporters, repeating talking points he’d been using on cable news shows in the wake of the New York election.

Asked how far the DCCC would go to compete in Ryan’s district, Israel said, “We’re going to continue to work closely with [Zerban], and we’ll make assessments as we go forward, but he is one of our first, early recruiting successes.”

It’s unclear how much help Zerban will need. In addition to a local finance director and spokesman, he has already hired a national media consultant and pollster. And he is believed to have the ability to self-fund, although he is raising money so far.

Zerban sold his contract food service companies two years ago and now serves as a Kenosha County supervisor, an elected position he first won in 2008. He declined to say how much he might be willing to invest in the race.

Zerban has limited connections to the political world. He made his mark in the food services industry, having been educated at the Culinary Institute of America. He started a business in 1992, which grew from 12 to 45 employees before it was sold.

Asked about his political leanings, Zerban acknowledged that he became a registered Democrat after moving to Wisconsin in 2004.

“I used to identify myself as a Republican,” he said, although he couldn’t say for sure whether he was ever registered as such. He attributes his change of party to the influence of his wife and a better understanding of public policy.

Zerban flirted with a Congressional run in 2010, ultimately deciding against it in a year that Ryan cruised to victory with 68 percent of the vote. Ryan has earned at least 63 percent in every election since his first in 1998, when he captured 57 percent.

A spokesman for Ryan’s campaign did not respond to a request for comment.

But National Republican Congressional Committee spokesman Paul Lindsay said the GOP welcomes the challenge.

“If Democrats want to use Paul Ryan’s district as a test model to talk about their plan to bankrupt Medicare, it’s tough to think of anyone better to challenge them on it,” he said. “Not only has Congressman Ryan weathered numerous election cycles in a swing district, he has the ability to hold Democrats accountable for threatening seniors’ access to treatment with their proposed rationing board of unelected bureaucrats.”

Although some liberal groups are expected to play in the district, the reality is that redistricting will be the ultimate factor in deciding how competitive Ryan’s race is.

It’s likely that Republicans who currently control the state Legislature will shift district boundaries to make the 1st more favorable for the GOP.

President Barack Obama carried the 1st district in 2008 with 51 percent, while President George W. Bush won it in 2004 with 54 percent.

“Redistricting may have a big impact on Paul Ryan’s Congressional district,” said Ali Lapp, executive director of the new Democratic political action committee House Majority PAC. “We’re definitely watching it. If it is a competitive district, we’ll certainly consider playing there.”

Beating Ryan, or even just giving him a tough race in 2012, would be considered payback for Democrats. National Republicans made a concerted effort to recruit top-tier challengers to Democratic chairmen in the 2010 cycle, and they ultimately knocked off three — including then-Budget Chairman John Spratt (S.C.).

Zerban is very serious about his chances, but he isn’t sure how seriously his opponent is taking him.

He recalled that he bumped into Ryan on a flight to Washington, D.C., in early May.

“I said, ‘I think I’m going to be your opponent in this next cycle.’ He said, ‘I look forward to running against you, if you make it through your primary,’” Zerban said, laughing. “I said, ‘You know, Paul, I don’t think I’m going to have a primary.’”

Joshua Miller contributed to this report.