Floridians in the state's 13th congressional district head to the polls Tuesday to vote in a special election, as national Democrats and Republicans eagerly watch for new voting trends in this high-stakes midterm election year.

The contest is between Democrat Alex Sink and David Jolly, a Republican, to fill the seat left open when Rep. Bill Young, R-Fla., the longest-serving House Republican, died in October at 82.

Jolly worked as an aide to Young, and Sink ran an unsuccessful campaign for Florida governor in 2010.

If that sounds like a run-of-the-mill congressional-race match-up, it's not. With eight months until an election that will decide what party controls the Senate and could partly reshape the House, Republicans and Democrats alike see the race as a testing ground for political messaging and a chance to weigh the potency of Obamacare as an electoral issue.

A victory for Republicans in the Democratic-leaning district could indicate that attacks on the president's signature health care law will remain a powerful weapon through this year.

If Democrats and Sink are able to win Tuesday, they might have found a spending-and-messaging formula that can be replicated elsewhere to beat back GOP criticism of the health law. Sink's message on health care, that Republicans would seek to repeal the entire law rather than fix its problems, is one that is currently popular among other Democratic candidates.

The proxy battle has attracted significant campaign money to the race. The candidates and outside groups have spent more than $9 million on ads, with Democrats and Sink outspending Republicans and Jolly.

Republican groups such as the Chamber of Commerce, American Crossroads, and the American Action Network have stepped up on behalf of Jolly, while the Democratic House Majority PAC has been a major spending force for Sink.

To manage expectations, Democrats have tried to downplay their edge. Florida's 13th congressional district narrowly favored Obama in 2012, but turnout for Democrats tends to slump in non-presidential-election years. DCCC Chairman Steve Israel publicly noted that the district strongly favors Republican candidates by as many as 13 points in off-years.

"This is a very tough terrain for us," Israel said in a recent interview with MSNBC.

Regardless of whether Democrats win, the party maintains little optimism that it will wrest back control of the House this year. Instead, Democrats' more pragmatic goal is to chip away at the Republican majority, positioning Democrats to be competitive during the 2016 presidential election, when more of their voters tend to show up to the polls.

The special election in Florida on Tuesday could offer a preview of whether the latter goal is a tenable one — or whether Democrats could be in for a rougher midterm election year than they had anticipated.