DES MOINES, Iowa – American restaurant chains and retailers famously use Columbus, Ohio as a test market for new products. The thinking goes that if something can’t sell in middle-class Middle America, it’s not a good risk.

Which, of course, is what brings us to Iowa.

Unlike their Republican counterparts in the Hawkeye State who tend to use their first-in-the-nation status to prop up doomed but ideologically pure candidates, Iowa Democrats have frequently been dream killers for their party’s left wing.

Bernie SandersHoward DeanGary Hart and Ted Kennedy all saw their insurgent bids rebuffed by Iowa caucus goers. But what keeps the candidates coming back is that sometimes, Iowans back longshots. And those longshots have come in big.

Both Barack Obama and Jimmy Carter owe their underdog nomination wins, and by extension, their presidencies, to their victories in Iowa.

So is it more that Iowa creates the momentum or that Iowa Democratic voters have tended to be pretty good judges of candidate horseflesh? It’s obviously some of both, but we tend to put more emphasis on the latter than the former. Over the past 48 years Iowans have rightly earned their reputations as skeptical but enthusiastic drum majors for the passing parade of the Democratic nominating process.

With that in mind, let’s take a tour through five very different Iowa counties that may make the difference for Democrats this year.

It has become fashionable in some quarters of the Democratic Party to complain about how white Iowa is compared to the rest of the country. Aside from the fact that whiteness itself is considered problematic in some of these same quarters, the argument goes that Iowa is not a good test of what voters in other states may want in a nominee. But they certainly can’t say that about Polk County. On age, income ethnicity, education and almost everything but their affinity for extremely thin pork loin sandwiches, the Des Moines metropolitan area looks a great deal like the nation as a whole. Like a lot of state capitals, it skews more Democratic than the rest of the state, but not wildly so. That all adds up to good news for Joe Biden and other more moderate candidates. Polk County isn’t just by far the biggest trove of delegates in the state, it’s tended to be a bellwether for Democrats, mirroring the final state result in the past two contested nomination races.

Population - 487,204

Adults age 25 and over with college degree - 36.1%

Median household income - $66,044

Non-white residents - 15%

Residents age 65 and older - 13.1%

General election results - 2016: Clinton 51.7%, Trump 40.4%; 2012: Obama 56.1%, Romney 42%

Dem caucus results - 2016: Clinton 53.1%, Sanders 46.1%; 2008: Obama 39.3%, Edwards 28.6%, Clinton 27.7%; 2004: Edwards 39.6%, Kerry 35.4%, Dean 14.2%

Did you know? - The Fort Des Moines Army base just south of the city was home to the first African-American officer candidate training class in American history. The facility, built as a cavalry base, was converted for World War I to an officer school for black candidates, commissioning its first officers in 1917.

Davenport is a proudly blue collar, union and Democratic city. John Deere, the Army arsenal on the island in the Mississippi between the city and Rock Island, Ill., Arconic Aluminum and other big employers make Scott County and the rest of the Quad Cities one of America’s major manufacturing hubs. That has helped turn the county into a Democratic stronghold. No Republican presidential candidate has won here since Ronald Reagan. But what kind of Democrats are they? They had been pretty moderate, but voters surprised race watchers in 2016 when they gave the nod, albeit narrowly, to Bernie Sanders. But maybe it shouldn’t have been a shock. The older, blue-collar voters here are a natural constituency for Sanders. But this time around, he’s got serious competition. Joe Biden has been a favorite of many union members and he has made a serious push in the Quad Cities. If Biden can show strongly here, it would be a good sign for his chances overall.

Population - 173,283

Adults age 25 and over with college degree - 31.9%

Median household income - $58,803

Non-white residents - 16%

Residents age 65 and older - 16.2%

General election results - 2016: Clinton 46.9%, Trump 45.4%; 2012: Obama 56.1%, Romney 42.4%

Dem caucus results - 2016: Sanders 50.6%, Clinton 49.4%; 2008: Obama 48.3%, Clinton 29.4%, Edwards 21.1%; 2004: Kerry 47.1%, Dean 22%, Edwards 19.1%

Did you know? - If your mother ever told you that something was “the best thing since sliced bread,” you can thank Scott County, Iowa. Davenport native Otto Frederick Rohwedder had his breakthrough in 1927 and eventually sold his patent rights to the miracle machine to Micro-Westco Co. in nearby Bettendorf. He worked there until his retirement nearly two decades later.

If you’re looking for the progressive voters of Iowa, look no further than Johnson County, teasingly called the “People’s Republic” by Iowans. Johnson County boasts the highest percentage of active Democratic voters of any county in Iowa. For Democratic candidates this county is a must because of its younger population. With the University of Iowa in Iowa City the median age is 29.9 years. Students turned out in large numbers for Bernie Sanders in 2016 and are expected to turn out again. But with Elizabeth Warren vying for the same progressive vote, it will test if Sanders has maintained his 2016 following. But it’s not all Bernie bros. There are plenty of suburbs where more moderate Democrats live. Results are expected to look different in those precincts, but organizers don’t expect serious traction for centrists. We can expect Johnson County to be the place where Sanders or Warren gain an edge over the other.

Population - 151,260

Adults age 25 and over with college degree - 53%

Median household income - $61,640

Non-white residents - 17%

Residents age 65 and older - 11.7%

General election results - 2016: Clinton 65.3%, Trump 27.4%; 2012: Obama 66.7%, Romney 31.2%

Dem caucus results - 2016: Sanders 59.8%, Clinton 40.2%; 2008: Obama 52.3%, Edwards 24%, Clinton 20.5%; 2004: Kerry 35.3%, Dean 28.7%, Edwards 26.3%

Did you know? - Johnson County’s most famous resident is painter Grant Wood, the creator of “American Gothic,” perhaps America’s most famous painting. The iconic 1930 portrait of an Iowa farmer and his daughter (really Wood’s dentist and sister) in front of their clapboard house with a Gothic window has become one of the most recognizable images in the world.  

Woodbury County’s courthouse is considered to be a Midwestern architectural masterpiece – a Prairie School gem finished in 1918. It draws architecture geeks from around the country. But what brings candidates to Sioux City every four years isn’t a pile of bricks but rather the largest pile of voters in the western part of the state. Woodbury County is a quintessential swing county. Between the years of 1988 and 2012 no candidate won the county by more than 3.5%. In 2016, however, President Trump won the county by nearly a 20% margin, his margin being the largest since Lyndon Johnson's election in 1964. Populists on the other side hope that what worked for Trump in the general election will work for them on caucus night. Bernie Sanders and Elizabeth Warren need to score big in this otherwise traditionally Republican region.

Population - 102,539

Adults age 25 and over with college degree - 23%

Median household income - $55,483

Non-white residents - 13.1%

Residents age 65 and older - 15.1%

General election results - 2016: Trump 56.6%, Clinton 37.1%; 2012: Obama 49.5%, Romney 48.5%

Dem caucus results - 2016: Sanders 52.8%, Clinton 47.2%; 2008: Clinton 36.5%, Obama 35.6%, Edwards 27.6%, Clinton 29.5%; 2004: Kerry 41.4%, Dean 28.3%, Edwards 22.5 %

Did you know? - The airport located in Sioux City, Sioux Gateway Airport, has the airport designator “SUX” which used to be quite the controversy. Advocates tried to change the identifier over the years, but ultimately decided to embrace it. To this day the airport sells merchandise that says “Fly SUX”.

Dubuque County is historically Democratic, but in 2016 President Trump was the first Republican presidential candidate to win this county since 1956, when Dwight Eisenhower was on the ballot. Dubuque is Iowa’s seventh-largest county, and in previous years it has given Democrats solid victory margin of about 10,000 in 2008 and 7,000 in 2012. The county’s economy is largely driven by manufacturing, though it has expanded to other areas like gambling, health care and education. Lessons from 2016 show Democrats need to reconnect with blue collar workers and rural voters as well as improve youth voter turnout. An opportunity for a Democratic win in Dubuque County could come from the growing suburban communities which tend to have strong voter turnout. Democratic voters here tend to be more moderate than other parts of the state, offering hope to Amy Klobuchar from neighboring Minnesota. But they’re pragmatic too, meaning this may be a place for Joe Biden to pad his numbers.

Population - 96,854

Adults age 25 and over with college degree - 30.5%

Median household income - $61,321

Non-white residents - 7.2%

Residents age 65 and older - 18%

General election results - 2016: Trump 47.2%, Clinton 46%; 2012: Obama 56.5%, Romney 41.8%

Dem caucus results - 2016: Clinton 52.1%, Sanders 47.9%; 2008: Obama 36.1%, Clinton 31.1%, Edwards 30%; 2004: Kerry 44%, Edwards 21.7%, Gephardt 17.7%, Dean 14.3%

Did you know? - “If you build it, he will come.” Recognize that line? Dubuque County’s claim to fame is that it is home to the town Dyersville, Iowa, which is where the 1989 movie “Field of Dreams” was filmed. The film is an adaptation of the book “Shoeless Joe,” written by William Kinsella. Visitors can visit the “Field of Dreams” movie site which includes tours of the century-old Lansing family farm and the baseball field.

Chris Stirewalt is the politics editor for Fox News. Brianna McClelland contributed to this report. Want FOX News Halftime Report in your inbox every day? Sign up here.