ROCHESTER, NH - OCTOBER 09:  Democratic presidential candidate, former Vice President Joe Biden attends a campaign event on October 9, 2019 in Rochester, New Hampshire. For the first time, Biden has publicly called for President Trump to be impeached. (Photo by Scott Eisen/Getty Images)
Washington CNN  — 

Former Iowa Gov. Tom Vilsack, who was the longest-serving Cabinet secretary in the Obama administration, is poised to endorse former Vice President Joe Biden’s presidential campaign on Saturday, four Iowa Democrats familiar with the matter tell CNN.

Vilsack, who served as agriculture secretary for eight years under President Barack Obama and Vice President Joe Biden, is set to back Biden’s candidacy at an event Saturday in Des Moines, the sources said.

Vilsack, a former two-term Iowa governor, is among the most respected Democratic leaders in Iowa. His wife Christie Vilsack, a former congressional candidate who is a prominent party leader in her own right, is also set to join the endorsement, the sources said.

The Associated Press was first to report the Vilsack endorsement.

The Vilsacks hosted Biden at an event earlier this year at their home, but they have also been attending events of several candidates. The expected announcement on Saturday comes as Biden is trying to regain his footing in Iowa after being overtaken in polls by Pete Buttigieg.

A CNN/Des Moines Register Iowa poll released last week showed Biden locked in a fight for second place with Massachusetts Sen. Elizabeth Warren and Vermont Sen. Bernie Sanders, with Buttigieg holding a clear lead.

A Biden spokesman declined to comment, but the campaign teased earlier this week a special guest would be attending a Saturday rally in Des Moines.

CNN’s Arlette Saenz contributed to this report.