US Senate Bernie Sanders on Friday slowly is narrowly leading in the Democratic presidential primary field in Iowa, with state’s party caucuses kicking off the 2020 presidential nominating calendar next month, according to a new poll.

The Des Moines Register/CNN poll, widely considered the most authoritative in Iowa, revealed that Sanders has garnered the support of 20 per cent of likely Democrat caucus-goers, Xinhua news agency reported.

Sanders, 78, is trailed closely by another progressive, Senator Elizabeth Warren of Massachusetts, who registered 17 per cent in the survey.

Last year, Sanders released a decade of tax returns, providing new insight into how the Democratic socialist from Vermont became a millionaire between his two presidential runs.

According to the returns provided by his campaign on Monday, Sanders and wife Jane’s bottom line jumped from $240,622 in 2015, the year he launched his first White House bid, to $1,073,333 a year later, as the once-obscure lawmaker became a political sensation on the left and a bestselling author with royalties pouring in, CNN reported.

Senator Amy Klobuchar of Minnesota notched 6 per cent in the poll, putting her in the fifth place, while entrepreneur Andrew Yang finished just below her with 5-per cent support.

Sanders reported a total 2018 family income of $566,421 — $382,920 of which came from writing and royalties. The documents showed he paid $137,573 in federal taxes in 2018 and owed $8,267 in taxes for the year.

The poll surveyed 701 likely Democratic caucus-goers in Iowa from January 2-8.

The poll is good news for Sanders, who increased his standing by 5 percentage points since the last Des Moines Register poll in November 2019. That survey showed Buttigieg surging to the front of the pack with 25-per cent support.

In 2016, during his first presidential campaign, Sanders released only one year of records — from 2014.

(With inputs from agency)