Vermont Sen. Bernie Sanders, who won Minnesota's last Democratic presidential caucuses, isn't ceding the state's 2020 primary to Sen. Amy Klobuchar.

There are early signs of enduring Sanders strength: More than 30,000 Minnesotans have signed up to support him since he joined the race Feb. 19, said Claire Sandberg, his campaign's national organizing director.

At least 81 house parties for those volunteers will be held on April 27 across Minnesota — in all eight congressional districts — as part of a nationwide launch of organizing efforts, she said.

That's "not surprising, given the level of grassroots enthusiasm in Minnesota" even before Sanders became a 2020 candidate, Sandberg said. It's a sign that Klobuchar's candidacy hasn't deterred the Vermont senator and several other Democratic challengers from investing resources in her home state.

Klobuchar, who is serving her third Senate term after a resounding statewide win last November, has thousands of volunteers in Minnesota, her campaign said in a statement. They're also helping her campaign in Iowa, which holds caucuses on Feb. 3, 2020.

Her presidential bid "is a grassroots effort that depends on people talking to neighbors, making phone calls and knocking on doors, which is what Sen. Klobuchar has done in previous campaigns and what we're building in states across the country," the campaign's statement said.

Sanders routed Hillary Clinton in Minnesota's 2016 caucuses, 61% to 38%. He held a rally Friday in Madison, Wis., but hasn't scheduled a campaign stop in Minnesota.

The state is returning to an open primary system and will be among at least 12 states to vote on Super Tuesday, an influential multistate primary on March 3.

Other Democratic 2020 presidential hopefuls, including former HUD Secretary Julián Castro, entrepreneur Andrew Yang and South Bend., Ind., Mayor Pete Buttigieg, have nascent operations here.

Sanders' is the most extensive, thanks in part to Our Revolution, a nonprofit group formed by staffers and volunteers after his 2016 campaign. Its state and local chapters, including several in Minnesota, have remained active. Other groups formed to draft Sanders to run again.

Eight unofficial gatherings organized by Sanders volunteers in Minnesota were held in January, Sandberg said.

"While Minnesota will be challenging for anyone not named Klobuchar, if she's still in the race by the time we vote … next year, I fully expect Bernie to come in a strong second place here," said Shawn Olson of Alexandria. He's a Sanders supporter who volunteered on Klobuchar's 2012 Senate campaign and ran for the state Senate in 2016.

There's little evidence of Minnesota campaign intentions by other high-profile Democratic candidates, including California Sen. Kamala Harris, Massachusetts Sen. Elizabeth Warren or former Vice President Joe Biden, who is considering joining the sprawling field.

Former Texas Rep. Beto O'Rourke has pockets of support here that have surfaced on social media. Fans of Hawaii Rep. Tulsi Gabbard met this month in Minneapolis to discuss ways to build interest.

Minneapolis City Council Member Alondra Cano and state Rep. Carlos Mariani, D-St. Paul, are encouraging Castro, a former San Antonio mayor.

"My role has been about how do I help strengthen the organizing behind his campaign," Cano said. She's working to "elevate the role of underserved communities" in presidential politics and expand support for Castro among Latino elected officials.

Cano has recorded Spanish-language radio ads to promote Castro's candidacy in Iowa. He has promised to campaign in all 50 states, she noted.

Yang, who appeared at a meet-and-greet session in Minneapolis in January, is returning for a May 5 rally at Boom Island Park.

Jamie Cravens, a "Yang Gang" organizer who lives in Arden Hills, said his Facebook group has more than 100 Minnesota members, and monthly meetings are planned. He's in touch with a loose network of Yang fans across the country.

Cravens believes that "a lot of people will gravitate toward [Yang's] more progressive ideas." Klobuchar's approach, he said, is "more pragmatic. I don't find it very inspiring for the most part."

Bloomington residents and small-business owners Donnie and Marla McFall are trying to drum up enthusiasm for Buttigieg. "I want people to know and get to understand him and what he's about and how he's a refreshing change," Marla McFall said of their unofficial efforts on social media.

Her husband, who backed Sanders in 2016, likes Buttigieg's "measured approach … and welcoming tone," which he believes could catch on across Minnesota. "I think he could go all the way," he said.

Sandberg said the Sanders campaign is looking beyond the primary to the general election. President Donald Trump lost the state narrowly to Clinton in 2016.

"Winning Minnesota this time is crucial," she said. "We need to make sure it doesn't flip to Donald Trump."

Judy Keen • 612-673-4234