Polls show a lead for Senator Sanders and former Vice President Biden.

On Monday, Iowa will host democratic Caucuses, a gathering of party supporters that will be the first test of voter preferences before the November election, and candidates are making a final effort to mobilize those who support them.

The Caucuses involve 11 candidates, each of whom hopes to become the official candidate of the Democratic Party and prevent the re-election of President Donald Trump for a second term.

Polls conducted the day before showed that Senator Bernie Sanders and former Vice President Joe Biden are leading, showing almost identical results. Senator Elizabeth Warren and former mayor of South Bend in Indiana, Pete Buttigieg, are trailing by a narrow margin.

But many Caucus participants say they are still undecided, and analysts admit some surprises.