Updated

President Obama's health care law was facing its first electoral test of the midterm year as candidates in a Tampa-area House race squared off Tuesday in a contest that both parties have spent millions on to win.

The candidates are Democrat Alex Sink and Republican David Jolly, and their contest is to succeed the late GOP Rep. Bill Young. The race is considered a tossup, and has quickly been cast as a political bellwether, and a testing ground for each party's messaging strategy -- which revolves in part around the Affordable Care Act.

Jolly said that if he wins, "I think there's no question Republicans hold the House in November, but I also think that means we take the Senate."

Jolly, a former aide to Young, has along with Republican groups spent millions to hammer his Democratic opponent over ObamaCare.

Sink, who narrowly lost the 2010 governor's race to incumbent Gov. Rick Scott, has cautiously embraced the health law -- while insisting it urgently needs fixing. She is playing down its importance in the special election.

"I hear a number of different issues that people are concerned about -- like protecting Social Security and Medicare," she said. "They're frustrated with Washington, believe that Washington is not working for them."

The perception of what the race means has inspired both parties to call in star advocates like President Bill Clinton and former vice presidential candidate Paul Ryan, in addition to blanketing the district with ads, calls and mailings. More than $11 million has been spent on the race, according to the Sunlight Foundation, a nonprofit group that tracks government information. More than one in four registered voters in the district is older than 65, a population that could account for more than half of those casting ballots.

The battle for Florida's 13th District seat is a prequel of sorts to the national fight this year over who controls Congress in the last two years of Obama's final presidential term. The House is expected to remain under Republican control. But in the Senate, Republicans are hoping to leverage Obama's unpopularity and his health care law's wobbly start to gain the six seats required to control the 100-member chamber.

That makes the race in Florida a pricey proving ground for both parties, with the candidates the faces of the effort.

Jolly, backed by Republicans and outside groups, says Sink would undermine Medicare because of Democratic-passed cuts to programs under the health law.

Sink and her allies, meanwhile, paint Jolly an extremist who wants to privatize Social Security and gut Medicare.

Clinton recorded a phone call last week seeking local volunteers to help with Sink's campaign, and a half dozen House Democrats emailed fundraising appeals to their own supporters on her behalf. More than a third of Jolly's campaign contributions came from members of Congress.

Meanwhile, Ryan joined Jolly on a conference call with voters, while Kentucky Republican Sen. Rand Paul recorded a phone message for the GOP nominee aimed at supporters of Libertarian candidate Lucas Overby.

While Republicans held the congressional seat for four decades until Young's death last year, the district's voters favored Obama in the 2008 and 2012 presidential elections. The district is 37 percent Republican, 35 percent Democrat and 24 percent independent.

Sink has outspent Jolly by more than 3 to 1 on television advertising, though outside groups aligned with the GOP have helped narrow the overall Democratic advantage.

Fox News' John Roberts and The Associated Press contributed to this report.