DES MOINES, Iowa — The race to take on Donald Trump begins in earnest on Monday in Iowa with Democrats struggling to identify a clear-cut presidential challenger, as the crowded contest heads to a photo finish in the heartland state.

Liberal Senator Bernie Sanders and the more moderate former vice president Joe Biden, both in their seventies, are setting the pace days before the first-in-the-nation Iowa caucuses.

But the two front-runners' divergent political views suggest Democrats remain undecided on which path — revolution or realism — their party torchbearer should take as they battle to avoid a Trump re-election in November.

With many Iowa voters still undecided, the race remains fluid, bringing uncertainty to a contest watched closely by voters nationwide, and no doubt by Trump himself.

As 12 Democrats make their final pitches to voters, Sanders this week jumped into a narrow lead in Iowa polling over Biden, the 77-year-old national front-runner.

The vote in the sparsely populated, snow-covered Midwestern battleground serves as the starting gun for the long election slog ahead, when Democrats decide whether to nominate a far-left firebrand like Sanders, 78, or tack toward the political center.

Monday will likely be a springboard for the top-performing candidates who can claim all-important momentum heading into the race's next chapters: New Hampshire on Feb. 11 and Nevada Feb. 22.

Pressure to perform is intense: a poor showing in Iowa is likely to send a candidate packing.

A handful of hopefuls have a realistic shot at winning the state.

South Bend, Indiana's centrist ex-mayor Pete Buttigieg is polling in third in Iowa, followed by Senator Elizabeth Warren, 70, who more closely aligns with Sanders and who has lost some polling steam in recent weeks.

Pragmatist 59-year-old Senator Amy Klobuchar from neighboring Minnesota is a wild card looking to score a major upset, as is tech entrepreneur Andrew Yang, 45.

With polls fluctuating and the race tight, few observers are making definitive forecasts.

On Tuesday a center-left think-tank warned Americans against choosing Sanders, a self-described Democratic socialist, to challenge Trump.

If Democrats nominate Sanders, "Trump's odds of winning a second term go up dramatically, which is why Team Trump has labeled him their 'ideal' nominee," the group Third Way said in an open letter to Iowa Democrats.

Biden claims Trump is terrified of facing him in November, and the former vice president points to Trump's pressure on Ukraine last year to investigate Biden and his family — an accusation at the heart of Trump's impeachment — as evidence.

Trump's defense team repeatedly focused on Biden this week as it argued its case in the Senate impeachment trial, saying calls to investigate the Bidens were appropriate because Biden's son Hunter served on the board of a Ukrainian gas company accused of corruption.

"The whole impeachment trial for Trump is just a political hit job to try to smear me because he's scared to death to run against me," Biden told a campaign rally Tuesday in Muscatine, Iowa.

Whether Trump's anti-Biden effort will singe the candidate is yet to be determined.

But the impeachment process has weighed on Iowa in another way: Senators Sanders, Warren and Klobuchar are pinned in Washington during the trial, forcing them to campaign for Iowa votes with one hand tied behind their back.

Klobuchar made a point of high-tailing it back to Iowa Tuesday after the trial adjourned, seizing the overnight break to campaign in Council Bluffs.

Meanwhile others were barnstorming the state unencumbered by Washington duties.

They include Buttigieg, a 38-year-old gay military veteran, who was urging voters to "turn the page" — apparently not just from Trump, but the aging Democratic establishment too.

"If we want to win... we've got to be ready to do something different, and I think the biggest risk we could take is to try to recycle the same old approach," he told reporters in Ottumwa.

With everything to play for, campaigns have deployed thousands of volunteers to knock on Iowa doors in a bid to enlist support. Television advertising is incessant.

But Iowans are used to face-to-face intimacy with candidates, who spend countless hours personally courting individuals in the months-long run up to the vote.

Iowa Democrats participate in caucuses, meetings where candidates are selected through a cherished if convoluted process that includes sales pitches from a presidential hopeful's representatives, and sometimes extensive debate, before votes are cast.

Some Iowans "actually feel they have a moral responsibility to push the candidates to take positions and explain themselves," Iowa State University professor Steffen Schmidt said.

Caucus participants "take politics very seriously." — AFP