Former Iowa Gov. Tom Vilsack endorses Clinton

Then Sen. Hillary Clinton (D-NY) accepts the backing of former Democratic Iowa Governor Tom Vilsack outside of Greenwood Elementary school on March 26, 2007 in Des Moines, Iowa.

Washington (CNN)Former Iowa Gov. Tom Vilsack has formally endorsed Hillary Clinton, helping her lock in support in the Hawkeye State as the possibility of a run by Vice President Joe Biden appears more likely.

"On Feb. 1, 2016 I intend to proudly caucus for Hillary Clinton — plain and simple. Hillary Clinton is not new to the fight for middle class families. Throughout her public life she has consistently fought to expand and strengthen the middle class. I have no doubt she will continue that fight as president," Vilsack wrote Tuesday evening in an op-ed in The Gazette of Cedar Rapids, Iowa.
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Vilsack, who is President Barack Obama's agriculture secretary, served with Clinton during Obama's first term in office and spoke highly of Clinton before locking in his support.
Clinton has been struggling against a surge of support for Vermont Sen. Bernie Sanders in New Hampshire and persistent questions about her handling of State Department emails on a private server.
    But support for the erstwhile Democratic frontrunner remains strong in Iowa, according to a recent poll of Iowa Democrats by Suffolk University.