ADEL, Iowa — The assembled at the Country Lane Lodge looked a lot like the typical Iowa voter: old and white. But the Democrats of Dallas County who had gathered for their spring fund-raiser were brought to their feet by a lean man from Missouri dressed in standard-issue millennial garb: suit jacket, bluejeans, narrow tie and two-tone half boots.

The speaker, Jason Kander, 37, talked about his experience volunteering for deployment in Afghanistan after graduating from Georgetown Law School, how Democrats needed to reclaim the ideals of patriotism and courage, and his new job leading Let America Vote, an organization that fights voter suppression and gerrymandering. The work just happens to take him to states like Iowa and New Hampshire. He also has a popular podcast and a book about “everyday courage” coming in August.

What hasn’t he done? Win a significant office, unless you count serving as Missouri’s secretary of state. But that didn’t stop an Iowa reporter from asking him if he was running for president. “It’s something that people do keep asking me about,” Mr. Kander said.

This is the season of the long audition.

“We are in open mic night,” said David Axelrod, a former senior adviser to President Barack Obama. “We are not even into auditions yet. I think this is the time where people are just trying out their material.”

“After the midterms,” he added, “this will intensify very, very quickly.”

Mr. Kander, along with Representative Seth Moulton of Massachusetts and Mayor Pete Buttigieg of South Bend, Ind., are among a group of younger Democrats — all military veterans — who are making a generational claim on a party that many see as top-heavy with leaders and lawmakers in their late 60s and 70s.

The very notion of veterans in the Democratic Party was called into question this month when Kevin Nicholson, a former Marine and a Republican candidate for a United States Senate in Wisconsin, questioned their “cognitive thought process” and suggested that “to fundamentally protect and defend the Constitution” was a “conservative thing” to do.

Mr. Moulton and Mr. Kander were among several veterans who signed a letter this month demanding that Mr. Nicholson apologize, saying they were “extremely disappointed to see” that he would “not just disrespect our nation’s veterans, but crudely do so as a means to advance his own political career.”

The Democratic veterans’ time may not yet have come, but they are doing what budding national figures need to do, traveling the country, giving speeches to party organizations, building political networks, refining their message and building a following on social media.

“I think there’s a generational thing here that people are ignoring right now,” said Anita Dunn, a former communications director for the Obama White House.

Mr. Moulton, 39, enlisted in the Marines after graduating from Harvard with a degree in physics in 2001. He served four tours in Iraq, where he earned a Bronze Star, then returned to earn a joint master’s degree at Harvard in business and public policy. After winning his House seat in 2014, Mr. Moulton started a political action committee to help recruit veterans to run for office, an organization that has him traveling the country giving speeches and raising his profile.

In Washington, he has opposed Representative Nancy Pelosi, 78, as the House Democratic leader. “This is the time for a new generation of leadership in our party,” Mr. Moulton said.

Mr. Buttigieg, 36, who volunteered for military service in Afghanistan in 2013 after earning degrees at Harvard and Oxford, emphasizes the ways government directly affects people’s lives. He is much in demand at state party organization gatherings, and will be the keynote speaker at the Wisconsin Democratic Party convention on June 1.

“A lot of the political actions you will see from the millennial generation aren’t just a result of younger people being a little more left, but really thinking how these political choices are going to affect us personally,” Mr. Buttigieg said. “Obviously we are the generation that has done most of the fighting in Iraq and Afghanistan.”

“From a budget perspective,” he added, “we are the generation that is going to pay the bill” for the tax cut signed into law in December.

They are competing for attention with the party’s mandarin wing, which includes former Vice President Joseph R. Biden Jr., 75, and Senators Bernie Sanders, 76, and Elizabeth Warren, 68, who were shaped by the Vietnam era. There is also a tier of younger senators like Kirsten Gillibrand, 51, of New York; Cory Booker, 49, of New Jersey; and Kamala Harris, 53, of California; and mayors of larger cities like Eric Garcetti, 47, of Los Angeles, and Mitch Landrieu, 57, of New Orleans.

How far any of the three veterans go in politics will be another test of the power of generational change, which propelled John F. Kennedy in 1960 and Bill Clinton in 1992. Millennials have now passed baby boomers as the nation’s largest bloc of voters, but they have few representatives in government.

“One of them, and I don’t know which one, is going to light a spark and get further than any of us can think,” Ms. Dunn said.

Part of the journey is coming up with ideas that separate them from Democratic orthodoxy. Mr. Moulton, for instance, said Democrats had to do more than simply oppose President Trump.

“I think there will be a reckoning that America goes through after Trump,” Mr. Moulton said. He added: “A lot of Democrats will self-righteously claim victory and try to lord it over the Republicans and say, ‘See, we were right all along,’ and that will be bad for the country. What we really need is Democrats who can be leaders and also uniters who can help the country heal after Trump.”

Each of the men frames an appeal that they believe will play to party strengths on the coasts and in urban areas, but also in the traditional Midwestern battleground states.

“The Republicans have actually done a good job of tapping into the anxiety people are feeling,” Mr. Moulton said. “But it also shows that Democrats have tremendous potential in places that people are hurting by showing a real path forward.”

Mr. Kander, who lives near Kansas City, raised a common worry in the Midwest, that its young people do not return to their hometowns because they see so little opportunity.

“We’re Democrats,” Mr. Kander said. “We give a damn. How did we ever let people convince us that was a weakness?”

Mr. Kander’s travels have taken him to dozens of states, and he now delivers a polished version of a stump speech, complete with applause-ready lines.

“We understand that patriotism is not about making everybody stand and salute the flag,” Mr. Kander said as the more than 225 people in Adel stood to cheer. “Patriotism is about making this a country where everyone wants to.”

None of the men have a traditional résumé of a candidate for national office. But Mr. Trump may have rendered such qualifications unnecessary.

Instead, they emphasize their military service as evidence that they can connect with multiple constituencies.

“People who serve in the military get a different vocabulary for talking with other Americans in a way that is less and less true in civilian life,” Mr. Buttigieg said. “You have to spend time working and eating and living with people who have radically different life experiences from you and very different politics.”

Mr. Moulton, he said, brings his military service, but also his “impatience for change within the party.”

“He has been outspoken in challenging our current leadership,” Mr. Connolly continued. “I think his challenge is whether you can channel that into effective coalition building to forge something by way of a movement.”

Mr. Connolly was largely admiring of Mr. Moulton, but also underscored the friction that his bucking of party leadership has revealed.

Paul Begala, a Democratic strategist who helped fashion Mr. Clinton’s rise, said that Democratic aspirants should look at Jimmy Carter’s campaign in 1976, when he rose from being the relatively obscure governor of Georgia.

“What Carter did was he understood the moment,” Mr. Begala said. “There was such disgust from Nixon and the lack of ethics, and he ran counter to that, to the imperial president in style and Watergate in substance. I think that’s the model.”

But it may well be that Democratic voters will want the polar opposite of Mr. Trump, namely someone with ample experience at governing. No member of the House has been elected president since James A. Garfield, in 1880. No mayor or person running an outside political organization has ever been directly elected president.