NEW YORK (Reuters) - Possible Democratic presidential candidate Joe Biden will tour Iowa by bus for eight days starting on Saturday, the longest running of his campaign to try to rally voters personally in an important state where his popularity has plummeted in opinion polls.

Biden, a former vice president, is seeking the party's nomination to face Republican President Donald Trump in a presidential election in November 2020 but faces a huge number of contenders for the nomination, many of whom have better-funded campaigns.

Iowa is the first state in which primaries will be held on February 3, and thus is a key test of the candidates' position.

Tom Vilsack, a former governor and agriculture minister, said many Democrats in the state had not made up their minds about the candidates.

"In the end, they will look for a figure who knows how to move the country forward in a realistic and progressive manner as well," said Vilsack, who announced his support for Biden this month.

Opinion polls show Biden leading by more than a dozen points in the state.

Biden, 77, still holds the lead over potential candidates in US-wide polls.