Three days before the first-in-the-nation 2020 presidential electoral contest in Iowa, the outcome of the Democratic caucuses remains uncertain. In contrast, President Trump, who held a rally in Des Moines on Thursday, has virtually no opposition in the Republican caucuses.

Only 41 out of the 4,750 voting delegates to the 2020 Democrat National Convention are ultimately at stake in Iowa. But the small number of delegates up for grabs is far less significant than the perception of momentum and success that will attach to the campaigns of candidates who win or finish above expectations in the Hawkeye State.

On Monday evening, an estimated 170,000 Iowans or more are expected to participate in more than 1,600 Democratic caucus events around the state. Any candidate who does not receive the support of 15 percent of the attendees at a caucus in the first round will be eliminated from the evening’s contest, which means the second candidate choice of each attendee matters a great deal.

The two most recent polls suggest the momentum is with the Sanders campaign.

A poll released on Wednesday of likely caucus attendees by Iowa State/Civiqs shows Bernie Sanders in the lead with 24 percent support, followed by Elizabeth Warren at 19 percent, Pete Buttigieg at 16 percent, and Joe Biden at 15 percent. Another poll released on Wednesday of likely caucus goers by Monmouth University shows former Vice President Joe Biden with a narrow lead over Bernie Sanders, 23 percent to 21 percent.

Both polls identify the strengths of each candidate within key demographic groups. Biden does well with older voters, Sanders does well with younger voters.

The Monmouth Poll noted that “nearly half” of likely Democratic caucus attendees have still yet to definitively decide who they back:

Currently, 47% of likely Iowa caucus goers are firmly decided on their candidate choice. That hasn’t changed much from Monmouth’s poll two weeks ago when firm support stood at 43%. Nearly half (45%) say they are open to switching support on caucus night, including 13% who rate this as a high possibility, 23% a moderate possibility, and 9% a low possibility. Firm support for the top polling candidates ranges from 47% for Klobuchar, 48% for Biden, and 49% for Buttigieg to 55% for Warren and 58% for Sanders.

As Nate Silver pointed out at 538.com on Thursday, polls have not always been good predictors of the final outcome on caucus night in Iowa:

Polls at this point in the 2016 Republican race had Donald Trump at 31 percent, Ted Cruz at 26 percent and Marco Rubio at 14 percent. The actual results were Cruz 28, Trump 24, Rubio 23 — so while the polls nailed Cruz, they were 7 percentage points too high on Trump and almost 10 points too low on Rubio. Polls in the 2016 Democratic caucuses did much better. Hillary Clinton led Sanders 49-46 at polls at this point, and she wound up winning the caucuses 49.9 percent to 49.6 percent, although it was close enough that you could essentially call it a tie.

“So out of 11 races [since 1988], we have only three cases (the 2016 and 2000 Democratic caucuses and the 2008 Republican caucuses) where the polls at this point were more or less spot on,” Silver adds.

While four candidates – Sen. Bernie Sanders (I-VT), former Vice President Joe Biden, Sen. Elizabeth Warren (D-MA), and Pete Buttigieg–have been in the top tier since September, the positioning of those four has been fluid over time.

In September, polls showed Joe Biden in the lead. In October, it was Elizabeth Warren. In November, it was Buttigieg. Now, in January, Sanders has a narrow lead according to the most recent polls.

According to the Real Clear Politics Average of Polls as of Thursday, Sanders is in the lead with 23 percent, followed by Biden with 20 percent, Buttigieg at 15 percent, and Warren at 14 percent.

Two months ago, on November 26, Buttigieg was in the lead with 24 percent support, followed by Sanders at 18 percent, Warren at 17 percent, and Biden at 16 percent.

Three months ago, on October 25, Warren was in the lead with 22 percent support, followed by Biden at 19 percent, Buttigieg at 16 percent, and Sanders at 15 percent.

Four months ago, on September 17, Biden was in the lead with 27 percent support, followed by Warren at 20 percent, Sanders at 15 percent, and Buttigieg at 9 percent.

Since September, several candidates have fallen by the wayside–Sen. Kamala Harris (D-CA), Sen. Cory Booker (D-NJ), former Rep. Robert Francis “Beto” O-Rourke (D-TX), and former San Antonio Mayor Julian Castro among them–while one, Sen. Amy Klobuchar (D-MN), is now just below the top tier, according to recent polls. The remaining survivors–Andrew Yang, Tom Steyer, Tulsi Gabbard, have yet to break beyond low single digits.

A number of variables could influence the final outcome, such as the weather on Monday night, and whether or not the Senate impeachment trial wraps up by Friday or Saturday, as some media outlets suggest may happen.

Should the impeachment trial drag on into next week, three senators – Warren, Sanders, and Klobuchar- will only be able to campaign in Iowa on Sunday, having to return to Washington, D.C. on Monday to be in the Senate chamber for the trial.