LANSING, Mich. -- As a Michigan field organizer for Bernie Sanders' 2016 presidential campaign, Mike McDermott trained volunteers to knock on doors and call voters, helping the Vermont senator upset Hillary Clinton in a crucial Midwestern state.

But as the 2020 campaign heats up, McDermott is all-in for Massachusetts Sen. Elizabeth Warren, creating a Michigan for Warren PAC to raise early money for her efforts and promoting her campaign through a website and Facebook page. While he's still a Sanders fan, McDermott sees Warren as a fresher face who's more electable and doesn't have the baggage of a 2016 loss.

"It's really 1a and 1b for me," McDermott said. "With Warren, I think there's more crossover appeal. She doesn't have 2016 branded on her."

That sentiment represents the new challenge facing Sanders, who is in second place in most national polls behind Joe Biden. The former vice-president has eaten into Sanders' base with appeals to blue-collar union voters. But Warren is emerging as another threat, winning over voters such as McDermott with a raft of proposals that sometimes go further left than those backed by Sanders.

Warren and Sanders are vying to become the progressive alternative to Biden, a competition that's especially pivotal in the Midwest. The region is critical to Democratic hopes of regaining the White House in 2020, and Sanders' campaign wrote in an April memo that he's "by far the best positioned candidate to win" in three upper Midwest states that handed President Donald Trump the White House.

The central peril Warren poses for Sanders is her status as the fresher liberal face in the race, eager to demonstrate her energy with hours of post-town hall photo lines, according to more than a dozen interviews with Michigan voters last week. Sanders still draws bigger crowds than Warren, who recently promoted her economic agenda before nearly 2,000 people at Lansing Community College, but the pro-worker, anti-establishment brand he brought to 2016 is no longer his alone.

"I don't think, because Bernie Sanders did as well as he did in Michigan last time, that that means anything this time," said Lisa Canada, political director with the Detroit-based Michigan Regional Council of Carpenters union.

Alexandra Lee, a 35-year-old graduate student from Lansing, said she backed Sanders in 2016 but now plans to support Warren when Michigan holds its primary on March 10. That will be one week after "Super Tuesday," when several states hold their nominating contests and the largest number of delegates are up for grabs.

Lee said she thinks it's time for a woman to be president: "I still like Bernie a lot. But I'd like to have someone younger and not male."

During her Michigan appearances, Warren laid out a populist pitch that subtly echoed some of the U.S.-workers-first messaging that helped Trump overtake Clinton in much of the Midwest. She attacked giant corporations that market themselves as American but make most their products overseas.

"Those giant corporations, the more and more power they amass, understand this: They're not loyal to America, and they're not loyal to American workers," the 69-year-old former law professor said in Lansing. "They are loyal to exactly one thing: their own bottom line, their own profits."

Warren's economic plan calls for "aggressive intervention" to create U.S. jobs and benefit U.S. exports, a spirit that aligned broadly enough with the anti-globalization rhetoric Trump invoked in 2016 to earn kudos from Florida GOP Sen. Marco Rubio and conservative pundit Tucker Carlson.

But Warren's pro-union economic message also aligns with the case Sanders made in his Michigan victory over Clinton. The Vermonter continues to make worker protection in trade agreements a centerpiece of his campaign, alongside a $15-per-hour minimum wage and single-payer health care.

Sanders told CNN on Sunday that his plan to lower student loan debt would "in some ways probably go further than" Warren's, adding that "not only Senator Warren, but others have moved" toward his position, "considered to be pretty radical" in 2016.

The key difference between Warren and Sanders as they jockey for position as the primary's leading liberal -- with Biden still far ahead in polls -- may be their political liabilities.

Warren has carefully set herself apart from Sanders' self-identified democratic socialism, calling for stronger "rules" in the nation's market economy to help make it more equal. Sanders' supporters view him as the more proven commodity against Trump, pointing to polls that show him performing better against the president, but Warren is less well-known at this early stage of the 2020 primary, making it possible that she has more room to climb.

In the critical early-voting state of Iowa, a new CNN-Des Moines Register primary poll released Saturday found Sanders tightly clustered with Warren and South Bend, Ind., Mayor Pete Buttigieg, rather than holding the clear second-place status he's had in most national surveys.

For Michigan Democrats like Abdul El-Sayed, whose unsuccessful gubernatorial campaign last year won Sanders' endorsement, the presence of two strong progressives in the presidential race will ultimately mean "a far better candidate" than 2016. El-Sayed has not endorsed in the 2020 primary yet but noted that Sanders has "a profound amount of support in our state" thanks to his first presidential bid.

Even so, some of that support may not remain firm. Cruz Villareal, 67, a college writing tutor, supported Sanders in 2016 but said he's looking for "radical change" -- the same thing he was looking for in 2016 -- and this time it's Warren who can deliver as the less polarizing candidate.

"I think that Bernie can't win in the general, and I think they will fight him from the left and the right," Villareal said. "They will do to Bernie what they did to him last time. I think they're less likely to do it to Elizabeth."

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Schor reported from Washington. Associated Press writer Emily Swanson contributed to this report.