MJ Hegar, the leading United States Senate challenger in Texas, rides a Harley-Davidson in viral videos and has called herself a "motorcycle-riding Texas Democrat."

She is not a Democrat who promotes "Medicare for All." She mentions that during a narrow loss for a House seat last year, her campaign signs appeared in front yards that also had ones for Republicans. And she sees unintended consequences with White House candidates who say they'll decriminalize border crossings.

The concern, Ms. Hegar says, is "tying the hands of law enforcement."

It doesn't take a biker jacket to show that Democratic candidates in some key states in the 2020 Senate race aren't going along as much of the party's presidential field takes a hard liberal turn. Big gaps already are apparent in races in Texas, Iowa, and Arizona. The latest discordant voice is Amy McGrath of Kentucky, a former Marine combat aviator running against Senate Majority Leader Mitch McConnell, who recalled wincing while watching a sprint to the left unfold during the first Democratic debates on TV.

President Donald Trump has sought to exploit the ideological rift between Democrats and fire up his own base by assailing four liberal women of color in Congress who call themselves "the squad," suggesting they get out of the country as he leans on issues of race in his reelection bid.

Policy differences always exist inside the major parties, but to some voters the Democratic agenda could soon seem like a hopeless argument with itself. More than in years past, progressives are insisting that winning in a polarized political environment requires ambitious ideas, not hedged compromises.

Even in moderate or conservative states, "having the candidate who is campaigning on exciting ideas is the biggest thing that will elevate them," said Adam Green, co-founder of the Washington-based Progressive Change Campaign Committee, which has backed Sen. Elizabeth Warren of Massachusetts for president.

In Texas, party leaders say they are confident in Air Force veteran Ms. Hegar taking on Republican incumbent John Cornyn, but they also are not discouraging progressives from challenging her. The field organizer for former Rep. Beto O'Rourke's near-miss campaign for U.S. Senate last year is now trying to draft Cristina Tzintzun Ramirez, the leader of a progressive Latino group called the Jolt Initiative.

In Arizona, progressives were deflated when Rep. Ruben Gallego, a liberal member of Congress, decided not to run against Republican Sen. Martha McSally and cleared the way for retired astronaut Mark Kelly, a moderate. And in Iowa, Theresa Greenfield has won early endorsements that include the Democratic Senatorial Campaign Committee, but that has not deterred liberal rivals from entering the race.

Democrats need to gain at least three seats next year to capture the Senate majority, and the map is an uphill climb. GOP seats are at stake in 22 states, but Trump carried 20. The argument is over whether the better approach is bold liberalism or cautious centrism.

In some contested states, if the leftward presidential tilt continues, the party's nominee and Senate candidates could wind up contradicting on almost every major issue, from immigration and race to health care and education. Democratic consultants say that's not a problem now: most voters at this stage are only broadly listening to whether candidates are on the same team.

Activists say the party shouldn't worry that later, closer to the election, some states' voters might be confused over what Democrats actually want to do if elected.

"Running as a moderate is to misunderstand the electorate," insisted Ed Espinoza, a former Democratic National Committee political director who now heads the group Progress Texas. "People may not identify with parties, but people want candidates that stand for something."

It's not clear yet how many Senate Democratic primaries will feature progressive candidates against moderates but such contests are more likely in House races where launching a campaign costs less.

In Arizona, a top Senate target for Democrats, Mr. Kelly already has taken pains to distance himself from presidential contenders like Elizabeth Warren, Bernie Sanders, and Kamala Harris. He's come out against Medicare for All, saying millions of people like their private health insurance, and against the plan to curb climate change called the Green New Deal. He's aiming to follow Sen. Kyrsten Sinema who ended decades of GOP dominance in Arizona last year by staying clear of her party's left wing.

"I don't like people going to their political and partisan corners and not working across the aisle to get things done," Mr. Kelly told The Associated Press when he announced his candidacy in February.

Progressives are better positioned in Maine. The liberal group Justice Democrats, which boosted Rep. Alexandria Ocasio-Cortez in her rise to Congress, is backing activist Betsy Sweet to unseat GOP Sen. Susan Collins of Maine, who is one of Democrats' top targets. A more moderate candidate, Sara Gideon, Maine's Democratic House speaker, has the backing of Senate Democrats' campaign arm.

In Texas, O'Rourke drastically reset Democrats' expectations for victory in the state after coming within 3 percentage points of ousting Republican Sen. Ted Cruz last year. The stunning finish and national attention propelled his run for president.

Ms. Hegar makes clear she plans to run a different campaign. Her compelling personal story – as a Purple Heart recipient who did three tours in Afghanistan – and eye-catching ads helped make her a national fundraising machine in 2018 when she nearly won a heavily conservative congressional district near Austin. She has raised more than $1 million so far.

Ms. Hegar rejects party labels. She said she "didn't let people push me into saying things that I don't believe. And I think the voters saw that and they saw authenticity."

Gilberto Hinojosa, chairman of the Texas Democratic Party, said Ms. Hegar "may not be considered as progressive as some people believe she should be" but that she'll appeal to all kinds of people.

At a recent rally for Mr. O'Rourke back in his home state, Krystal Reilly said Ms. Hegar was where she needed to be.

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"Beto brought people out of the grave to get to those polls and it's a fight, right?" Ms. Reilly said. "If she goes any more left in the state of Texas, it's not going to work."

This story was reported by the Associated Press. Clarice Silber in Austin, Texas, Jonathan J. Cooper in Phoenix and Alan Fram in Washington contributed to this report.