Democrats from the Twin Cities’ southern suburbs gathered in a Prior Lake ballroom last Sunday with an odd emotion about Minnesota’s Republican-held 2nd Congressional District: hope.

“The tide is blue in CD2,” Democratic-Farmer-Labor Party Chairman Ken Martin declared in a speech.

Republican Rep. John Kline has long won comfortably in the 2nd District, which stretches from the southern suburbs south to Northfield, Zumbrota and Plainview. But Democratic President Barack Obama won the district in both of his elections, and now Kline’s retirement has Democrats believing in victory.

This liberal optimism stems from hard data: the leftward march of the Minneapolis-St. Paul suburbs. Areas once won regularly by Republicans became swing districts and now lean toward the DFL. This applies not only to the innermost ring of suburbs — many of which have become DFL strongholds — but to the second and even third ring of suburbs, which are all far more hospitable places for Democrats than they were during the Clinton administration.

Twenty years ago, Republican legislative candidates averaged more than 60 percent of the vote in cities such as Edina, Eagan, Bloomington and Minnetonka, according to a Pioneer Press analysis. Over the past three elections, all of those cities have averaged majority support for Democrats.

Cities such as Plymouth, Burnsville and Arden Hills are undergoing a similar shift but with a slower trajectory. Those three cities have averaged a very slight majority for Republicans in recent legislative elections but are trending leftward.

“I am feeling very positive about our prospects to push out further into the suburbs,” said House DFL leader Paul Thissen of Minneapolis.

Republicans acknowledge the trend but point to a silver lining for them: As Democrats have done better in the inner suburbs, Republicans have done better in the faster-growing outer suburbs. Cities such as Chanhassen, Lakeville, Ham Lake and even Prior Lake, where Democrats gathered last week, remain Republican strongholds. Outer suburbs like these have shown little growth in Democratic performance — and have even become more Republican.

“What used to be the exurbs are now becoming the suburbs,” said former House Speaker Kurt Zellers, from solid-red Maple Grove. “I think that bodes well for us.”

PART OF A NATIONAL TREND

Minnesota’s not unique in seeing its suburbs move toward Democrats. In most major metropolitan areas around the country, the trends are the same.

“You have the suburbs both growing and, on net, seeming to move slightly left,” said Ruy Teixeira, a political demographer at the Center for American Progress. “By and large, metros that are growing fast exhibit this pattern more clearly.”

In Colorado, Teixeira said, the Denver metropolitan area features Democratic-leaning inner suburbs and Republican-friendly exurbs.

But Wisconsin’s Milwaukee suburbs have shown almost no evidence of this trend. While Chicago, Philadelphia and the Twin Cities have seen left-moving suburbs, suburban Milwaukee is just as Republican as it was under President Ronald Reagan.

In Minnesota, the biggest political shifts have been in suburban Hennepin, Ramsey and Dakota counties, all of which have become more Democratic over the past decades. Anoka, Carver and Scott counties have seen far less evidence of any leftward trend, or have become even more conservative. Washington County shows mixed results, with some cities moving left and others right.

Teixeira said that each metro area is different but that, in general, experts distinguish between three types of suburbs:

— The inner, “urbanizing” suburbs often look very similar to the big cities they abut. In the Twin Cities, that includes places such as Richfield, Roseville and Brooklyn Center. These urbanized suburbs have dense populations and are moving to the left.

— Further out, “emerging” suburbs aren’t “quite as dense as the urbanizing suburbs, but are getting there,” Teixeira said. “They also tend to be moving to the left.” Bloomington, Eagan and Inver Grove Heights might fall into this category.

— On the outskirts of metropolitan areas are exurbs, such as Chanhassen, Maple Grove, Ham Lake and Lakeville. These tend to be quite conservative and are often staying that way.

The good news for Democrats in this breakdown is that more people live in the inner suburbs. So even though the exurbs often have higher growth rates, more people each year are moving to the left-leaning areas.

BEHIND THE SHIFT

One of the big questions is not whether the suburbs are moving left, but why. It could reflect new Democratic voters moving in, or existing Republican voters changing their politics, or a mixture of both.

Some Democrats argued that their party’s positions played a big role in their suburban gains.

DFL chairman Martin, for example, acknowledged some demographic shifts but said Democrats’ focus on education, transportation and other issues of interest to suburban dwellers played a part.

“We are talking about people who have deep concerns with those issues that we are talking about,” Martin said.

Minnesota Rep. Roz Peterson, whose legislative district straddles left-moving Burnsville and solidly red Lakeville, was the only suburban Republican to oust a sitting Democrat in 2014. She said suburbanites’ unique approach to political issues plays a big role in who wins elections.

“When the Republicans all got elected in 2010, (suburban voters) were like, ‘We can’t elect Democrats because we know they’ll raise our taxes and increase spending,’ ” Peterson said. But in 2012, she argued, “there’s those social issues that captured a lot of people” for Democrats.

That reflects a stereotype of suburban voters as fiscally conservative and socially liberal. Suburban voters tend to be further left on social than fiscal issues, but Teixeira said they’re increasingly voting with Democrats on the economy, too.

“Are they as liberal on economic issues, on the role of government, as people who live in the urban areas? Probably not,” Teixeira said of suburbanites. “But they’re still more liberal than the country as a whole.”

Teixeira said he believes that liberal city-dwellers are moving out into the suburbs and taking their politics with them.

For one thing, the Twin Cities suburbs are far more racially diverse than they were decades ago. From 1990 to 2010, racial minorities rose from 4.6 percent of the Twin Cities suburbs to 18.1 percent, according to U.S. census data. Democrats tend to do better than Republicans with minority voters. Another factor is a suburban increase in college-educated white voters, another group that leans left.

Development patterns can also have a strong impact on political leanings.

In a Pew Research study, a majority of conservative voters said they would rather live in big, spread-out neighborhoods, as are common in exurbs. Majorities of liberals preferred the small, walkable neighborhoods — common in and near big cities.

Brookings Institution senior fellow Robert Lang summarizes this trend in three words: “Density equals Democrats.”

Peterson said she’s seen development patterns change in Burnsville since moving there in 1986.

“At that time, Burnsville was mostly single-family homes. Now, we have a lot more multifamily housing,” Peterson said.

Burnsville has moved to the left over that time, while Lakeville, which hasn’t, is only just beginning to see major multifamily development, Peterson said.

WHAT THE FUTURE HOLDS

In cities like Apple Valley, longtime Democratic activists Roxanne and Dave Mindeman said Democrats sometimes believed they were all alone.

Like others, the Mindemans tell of door-knocking in the suburbs and homeowners quietly telling them that they voted for Democrats but were the only ones of that political persuasion. And hearing the same secret at the next house. And the next one.

Stories like that have Democrats optimistic that they can not only capitalize on the left-moving inner suburbs but expand the blue tide farther.

But House Speaker Kurt Daudt, R-Crown, said the GOP House has a “sustainable majority” despite the loss of the inner-ring suburbs.

“We are not defending in Edina and Eagan like we have in the past, and those are really expensive seats,” Daudt said.

Rural seats, which swung to Republicans last year, are cheaper to defend and simply contain more Republican voters.

But Daudt said he’ll fight for seats in the suburbs in 2016, too. In 2014, Republicans won big in rural areas and lost narrowly in the suburbs.

“We are going to play in the suburbs this year,” Daudt said.

In the long term, Teixeira said, suburbs will likely continue to become denser, more educated and more diverse. Those trends have benefited Democrats, but it’s no guarantee that same coalition will back Democrats decades into the future. If Republicans manage to effectively appeal to the emerging suburban population, they could halt or even reverse their suburban decline. Alternately, Teixeira said, the current leftward move could continue for a long time.

“No trend goes on forever,” Teixeira said. “(But) the underlying demographic trends, regardless of what they mean politically, are going to continue. You can bet on that one.”

Follow David Montgomery at twitter.com/dhmontgomery and follow Rachel E. Stassen-Berger at twitter.com/rachelsb.