The single county to watch in Montana's special election

— -- If you’re only watching one county in Thursday’s special election for Montana's lone U.S. House seat, it should be Lake County.

Wrapping around the base of Flathead Lake in rural northwest Montana, the county includes just three cities and towns, in addition to the Flathead Indian reservation and other rural areas. But this unassuming rural Montana area has had nearly perfect accuracy in predicting Montana's federal and gubernatorial statewide elections over the past two decades.

Republican multi-millionaire tech executive Greg Gianforte is slated to face off against Democratic populist singer-songwriter Rob Quist in this GOP-leaning U.S. House district on Thursday, after the seat was vacated by now-Interior Secretary Ryan Zinke.

Republicans have held this U.S. House seat for the last two decades and are expected to hold onto it this week, but Montana has been known to split their tickets: they have a sitting Democratic governor and U.S. senator. And even though President Donald Trump won the state by more than 20 percentage points, it's incumbent Democratic governor also won re-election in November.

But in November, Lake County pinpointed both candidates' support in both races within one percentage point. In fact, only once since 1996 has the county failed to vote with the winner of the statewide vote, according to an ABC News analysis of data from The Associated Press and the Montana Secretary of State.

The county did not match the statewide vote in the 2008 presidential race -- the only mismatch in federal or gubernatorial races in the last two decades -- siding with former President Barack Obama by a 49-47 percent margin while the overall state voted for Sen. John McCain by a 50-47 percent margin. The next closest election it missed? The U.S. Senate race in 1996.

Compare for yourself: Here's a look at each race over the last two decades.

Not only has Lake County called the correct winner with a shocking degree of accuracy; it's also precisely matched the support of each candidate in such statewide races.

Lake County has matched both major party candidates' statewide result within 2 percentage points or less in 22 of the last 26 federal and gubernatorial elections. It’s also predicted the statewide margin within an average of 2.5 percentage points since 2002 -- and a razor-thin 1.2 points in the last five such statewide races.

So what makes Lake County such a predictive swing county in Montana? It's home to fewer than 30,000 people in only three census-designated cities or towns.

Democratic-leaning areas are mostly located in the south of Lake County. The only precinct to vote for Hillary Clinton in the 2016 presidential race was in Arlee, a small town of 636 people dubbed the 'southern gateway' to the Flathead reservation. Half of its population is "American Indian," according to the census, a demographic which tends to vote Democratic. The former secretary of state defeated President Trump by a 50-43 percent margin there and it went for Democratic Gov. Steve Bullock by more than 30 percentage points.

Other small areas in the southern part of the county with large American Indian populations -- like St. Ignatius and Ronan -- also went for Bullock in the gubernatorial race in November. The eastern side of Polson -- the major city in Lake County located on the southern tip of Flathead Lake -- also went narrowly for Bullock.

But rural areas in the northern part of the county vote overwhelmingly for Republican candidates. One rural precinct near Swan Lake voted for Trump by almost 60 percentage points. Other strong GOP areas that voted for Gianforte in the gubernatorial race by double digits include Dayton, Ferndale and the western side of Polson.

Here's a look at the county's geography:

And if there's one precinct to watch, it's the eastern side of downtown in the county's largest city: Polson. The 5th precinct nearly exactly matched both the Lake County and the Montana statewide margins in these two races. Trump won the precinct by 19 percentage points and Bollock won it by a single percentage point. Its eastern neighbor, the 7th precinct of Polson, is another one to watch.

The county also splits its representation in the state legislature among five Republicans and three Democrats, according to the county's website.