The battle for the Democratic nomination is the most unpredictable in at least two decades, with just three weeks left before the Iowa caucuses.

There has been precious little winnowing of the field, at least since Sen. Kamala Harris Kamala HarrisScott Walker helping to prep Pence for debate against Harris: report California family frustrated that governor, Harris used fire-damaged property for 'photo opportunity' Moderna releases coronavirus vaccine trial plan as enrollment pushes toward 30,000 MORE (D-Calif.) dropped out in early December.

Four candidates have a plausible shot at winning the Iowa caucuses and the nomination itself: former Vice President Joe Biden Joe BidenCast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response Biden tells CNN town hall that he has benefited from white privilege MORE, Sens. Bernie Sanders Bernie SandersMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Why Democrats must confront extreme left wing incitement to violence MORE (I-Vt.) and Elizabeth Warren Elizabeth WarrenWarren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon No new taxes for the ultra rich — fix bad tax policy instead MORE (D-Mass.), and former South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBogeymen of the far left deserve a place in any Biden administration Overnight Defense: Woodward book causes new firestorm | Book says Trump lashed out at generals, told Woodward about secret weapons system | US withdrawing thousands of troops from Iraq A socially and environmentally just way to fight climate change MORE (D).

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Further roiling the field are two billionaires, environmentalist Tom Steyer Tom SteyerTV ads favored Biden 2-1 in past month Inslee calls Biden climate plan 'perfect for the moment' OVERNIGHT ENERGY: Trump administration finalizes plan to open up Alaska wildlife refuge to drilling | California finalizes fuel efficiency deal with five automakers, undercutting Trump | Democrats use vulnerable GOP senators to get rare win on environment MORE and former New York City Mayor Michael Bloomberg Michael BloombergTop Democratic super PAC launches Florida ad blitz after Bloomberg donation The Hill's 12:30 Report - Presented by Facebook - Latest with the COVID-19 relief bill negotiations The Memo: 2020 is all about winning Florida MORE, whose extensive spending on television advertising has enabled them to make inroads.

Bloomberg has risen to around 6 percent support in the RealClearPolitics national polling average, while the political world was startled by two Fox News polls released Thursday that showed Steyer at 15 percent support in South Carolina and 12 percent in Nevada.

That’s not the only complicating factor.

Speaker Nancy Pelosi Nancy PelosiMcConnell focuses on confirming judicial nominees with COVID-19 talks stalled Overnight Defense: Top admiral says 'no condition' where US should conduct nuclear test 'at this time' | Intelligence chief says Congress will get some in-person election security briefings Pelosi must go — the House is in dire need of new leadership MORE (D-Calif.) has signaled that she will send articles of impeachment to the Senate next week, a move that will require the upper chamber to start its trial of President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE almost immediately.

That means senators such as Sanders, Warren and Amy Klobuchar Amy KlobucharEPA delivers win for ethanol industry angered by waivers to refiners It's time for newspapers to stop endorsing presidential candidates Biden marks anniversary of the Violence Against Women Act, knocks Trump and McConnell MORE (D-Minn.) will have to divide their time between Washington and Iowa even as the final sprint to the caucuses is underway.

The nomination battle this year has a fluidity that was absent in 2016, which was effectively a two-horse race between Sanders and eventual nominee Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE, and in 2008, when only three candidates — then-Sen. Barack Obama Barack Hussein ObamaThe Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon Trump appointees stymie recommendations to boost minority voting: report Obama's first presidential memoir, 'A Promised Land,' set for November release MORE (D-Ill.), former Sen. John Edwards (D-N.C.) and Clinton — were in serious contention.

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The race is also being roiled by the aftershocks from President Trump’s decision to order the killing of Iranian Gen. Qassem Soleimani.

The controversy that followed has placed foreign policy at the center of a campaign that had previously been fought on domestic issues and the overarching question of which Democrat is best placed to defeat Trump in November.

In particular, the Iran crisis placed a new spotlight on Biden, whose supporters highlight his experience in foreign affairs but whose detractors note his 2002 vote, while a senator, to give then-President George W. Bush authority to go to war in Iraq.

Trump’s campaign, meanwhile, has turned its fire increasingly on Sanders, casting him as weak on national security. Team Trump’s moves suggest it is becoming worried by the threat posed by the Vermont senator, however.

Sanders came out on top of the keenly awaited Des Moines Register poll in Iowa, which was released Friday evening. The poll — the gold standard in the Hawkeye State — gave Sanders 20 percent support, narrowly ahead of Warren with 17 percent, Buttigieg with 16 percent and Biden with 15 percent.

The poll gave new optimism for Warren’s supporters after a period in which she had seemed to lose altitude. Conversely, it is a sign of alarm for Buttigieg, whose support in the state has dropped 9 points since a November survey by the same pollster.

The scrutiny Buttigieg has faced after an earlier surge may be taking its toll. A poor result in Iowa could also spell real trouble for the former South Bend mayor, who has signally failed to attract black support and is expected to struggle once the primary process moves to more ethnically diverse states such as fourth-to-vote South Carolina.

The idiosyncrasies of the Iowa caucuses, where supporters must gather at specific venues and submit to a more complicated process than simply casting a ballot, could help candidates who have an especially committed base. That could benefit Sanders.

“I think Bernie Sanders has advantages because he has done it before and his supporters are nothing if not devoted,” said Democratic strategist Tara Dowdell, who is not affiliated with any candidate. Dowdell noted that Warren has a similarly “passionate” following and that her organization in Iowa is said to be particularly strong.

Biden’s camp has been downplaying his chances of winning in Iowa, stressing his strength nationwide as well as with black voters who will be central to later primaries. Still, they haven’t given up on Iowa.

Dick Harpootlian, a member of the finance committee of Biden’s campaign and a state senator in South Carolina, told The Hill he would be traveling with friends to Iowa in the coming days to go door to door on behalf of the former vice president.

“If he were to win Iowa, that would dampen the enthusiasm for the rest of the field,” Harpootlian said.

But he also insisted that Biden is the strongest candidate to defeat Trump, win or lose in Iowa.

“It’s the only argument that anybody ought to be having right now: who can beat Donald Trump,” Harpootlian said. “Our worst person is better than him but they can’t win the swing states. I hear this idea that we have to ‘lead on issues’ or ‘show who we are.’ I’ll tell you who we are: We’re the folks who need to beat Donald Trump.”

The closeness of the race in Iowa makes the final televised debate before the caucuses particularly crucial. The debate will take place on Tuesday evening at Drake University in Des Moines.

The Iowa result can reset the race for the nomination at a moment’s notice, as it did when Obama won in 2008 and Clinton was relegated to third place behind Edwards, a shock from which her campaign never fully recovered.

That’s one reason why the candidates want to leave no stone unturned between now and the Feb. 3 caucuses.

“I still think Iowa and New Hampshire are going to set the table for the rest of the race,” said one Democratic strategist who requested anonymity. “After that, do all four of these people still have a chance of winning the race? And if not, which one is ascendant?”

The Memo is a reported column by Niall Stanage primarily focused on Donald Trump’s presidency.