In exactly four weeks, voters will choose their next member of Congress in Florida's 13th district. And around the country, the political world will be watching with a very close eye.

Democrat Alex Sink (AP Photo/J Pat Carter, File)

The race in the bellwether district has attracted widespread attention in recent weeks. It's shaping up as a harbinger of the midterms and is increasingly seen as a must-win opportunity for both parties.

So why is this race getting so much attention and what does it mean? Here's what we know about the contest between Democrat Alex Sink and Republican David Joly with a month left to go:

1. This is as good a midterm preview as we have seen so far this cycle.

The 13th district is as purple as they come. President Obama won in in 2012, but it was represented by long-serving Republican C.W. Bill Young until his death last year. If there's a natural testing ground for broad messages on Obamacare, lobbying, Social Security and the issues dominating the national political conversation, this is it. No special election is a perfect barometer of wider implications, of course -- thus the word "special" -- but this will be as  good a ballot box test as we have seen this cycle. Last year's House specials took place on highly partisan terrain and the governor's races in Virginia and New Jersey came before the broader problems with the rollout of Obamacare had really sunk in. And while polls are useful measuring sticks, they are no substitute for actual elections. One thing to note about the district that makes it different from a lot of others, though: It's residents are on average older than most places.

2. Both Democrats and Republicans still think they can win.

Surrogates and outside groups don't flood in when a race is one-sided. No one wants to be associated with a loser in politics. But the die has yet to be cast here, as shown by the likes of Vice President Biden, former governor Jeb Bush (R) and Sen. Marco Rubio (R-Fla.) pitching in. That's not to mention the flurry of outside groups on both sides -- House Majority PAC, both the DCCC and NRCC, and American Crossroads, to name a few. Watch to see if groups on either sides start cutting bait later for a sign of which candidate has the momentum.

3. National labels matter. 

Special elections can be referenda on parochial travails or the strengths of the candidates themselves and to an extent, that's true in this race. But it's also is shaping up heavily as a test of bigger themes and labels we might see pop up in other races. Just look at the ads both sides are running. Republican groups are doubling down on tying Sink to President Obama and Obamacare while Democrats are slamming Jolly over his lobbying past and Social Security. These are themes we're likely to see reprised in other places later this year. (Sink's record as former state CFO has also come under scrutiny in Republican ads.)

4. A loss would be bad for either side, but worse for Democrats. 

Given the amount of resources both parties are pouring into the campaign, there's no doubt there would be widespread disappointment for the side that loses. And if Republicans spend more, a loss would be especially painful for them. But think of it this way: Democrats recruited a strong fundraiser with good name recognition and a career spent outside Washington. Republicans settled on Jolly only after other top names passed. If Republicans are still able to pin the unpopularity of Obama and the health-care law on a Democratic dream candidate like Sink, think of that they can do to congressional Democrats this cycle. Remember, Democrats are racing to save an increasingly fragile Senate majority, while the GOP House majority seems all but completely secure.