By Agence France-Presse 

Democratic frontrunner Joe Biden went on the attack Thursday against Donald Trump, campaigning in Iowa hours before the US president jets in to divert attention ahead of the state's first-in-the-nation vote. Democratic presidential candidate, former Vice President Joe Biden  (Scott Eisen / Getty Images / AFP / MANILA BULLETIN FILE PHOTO) "I can hardly wait to debate this man," the former vice president told a crowd in Waukee, a whistlestop on his weeklong bus tour across the state. Candidates battling to challenge Trump in November's elections are locked in a tight race days before the caucuses in Iowa, where candidates are barnstorming the state, countless volunteers are knocking on doors seeking to sway undecided voters, and political advertising is carpet-bombing the airwaves. But while candidates are desperate to stand out against their rivals, occasionally clashing on policy, performance or personalities, Biden has pivoted directly towards the president, making the case for a Biden-Trump election. Trump is trying to "destroy" the healthcare law known as Obamacare, ignores the threat of climate change, has "walked away from our allies and embraced dictators and thugs," and is at risk of "starting a war with a tweet," the 77-year-old Democratic party elder added. Monday's vote is the effective starting line of a race that has endured a yearlong warm-up, with the largest and most diverse Democratic field in history seeking to identify a unified party vision that they believe can prevent a Trump re-election. Iowa's winner will pocket critically important momentum with the contest shifting to New Hampshire and then Nevada and South Carolina, the four early-voting states in the lengthy process. With a dozen candidates converging on Iowa to make their final pitches, the race is tight. Far-left Senator Bernie Sanders, 78, is leading the charge in the state, with Biden hot on his heels. Two more candidates, centrist former Indiana mayor Pete Buttigieg, 38, and progressive Senator Elizabeth Warren, 70, are within striking distance, with a fifth candidate, 59-year-old Senator Amy Klobuchar, in the second-tier but hoping to land a major upset. Their divergent political views suggest Democrats remain undecided on which path the torchbearer should take in the general election, and Trump is likely to mock Democrats' struggle to identify a clear-cut challenger.

- Fluid race -

Prior to jetting in to snowy Des Moines for a rally, he stopped in Michigan, a swing state that he surprisingly won in 2016. "This time should be easier because we've really produced," Trump told employees of a factory, who briefly chanted "four more years" as he boasted of a sweeping new trade agreement between the United States, Mexico and Canada. He also hammered Democrats over their efforts to remove him from office via the Senate impeachment trial that has infuriated the Republican base. The president listed some of his economic accomplishments, then added: "And what do they do? They impeach you! Explain that one." Despite the stain of impeachment, Trump's supporters outside his Iowa rally venue sang his praises and commended him for heading to the heartland as a challenge to Democrats. "I love him. I am so sick and tired of what the other party is doing to him and trying to take him down," retiree Linda Moon, 72, told AFP. "He isn't perfect," she added. "But he's doing really good things in this country." With many Democratic Iowa voters still undecided, the nomination race remains fluid. But the party's statewide chairman, Troy Price, said the large number of undecided voters so close to the caucus was due to "so many great candidates" running this cycle. "We may not have a good sense of the outcome until we start getting the results on caucus night," he told AFP. But there was broad agreement in opposition to Trump. "They're so ready for a change," Price said. "They're tired of the fighting, they're tired of the broken promises that this president has made." Complicating matters, Sanders, Warren and Klobuchar largely have been tied to Washington for Trump's trial, denying them crucial face time with fastidious Iowa voters. Warren boasts the best Iowa ground game and won the endorsement of the state's largest newspaper, but her campaign has failed to convert those advantages into greater support. Her surrogates, including her golden retriever Bailey, were left campaigning without her this week, seeking to win over undecided Iowa voters. Biden meanwhile rumbled past snowy cornfields on a "Soul of the Nation" bus tour, at each stop accusing Trump of smearing him because he was "scared" of running against Biden. Biden has been linked by name to the impeachment trial whose charges against Trump focus on the president withholding military aid from Ukraine last year until its leader committed to investigating the Biden family. Second-tier candidate Andrew Yang, a tech entrepreneur running an outsider campaign, was also criss-crossing Iowa on a bus tour, seeking to "shock the country" with a strong showing in the state.