Hillary Rodham Clinton has snagged a big endorsement as she brings her presidential campaign to the Iowa State Fair. 

Former Iowa Sen. Tom Harkin has endorsed Clinton's bid for the Democratic nomination, writing in The Des Moines Register that the former secretary of state is a "fighter who has a record of getting things done." 

Harkin says in the op-ed published online Thursday night that Clinton "has devoted her career to championing the needs of children" and has been a "tireless advocate for women and families." 

The endorsement kicks off a weekend of campaigning in Iowa, home of the nation's first presidential caucuses. Most of the Democratic presidential field, including Clinton, will address Democrats in northern Iowa at Friday night's annual Wing Ding fundraiser. On Saturday Clinton will attend the Iowa State Fair, a traditional stop for presidential hopefuls. 

Clinton is pouring staff and resources into Iowa's caucuses to fend off a strong challenge from Vermont Sen. Bernie Sanders, who has drawn large crowds in the state and around the country, and former Maryland Gov. Martin O'Malley, who is planning a major push into the state. 

Harkin did not seek re-election in 2014 after serving three decades in the Senate, but his endorsement has been coveted among Democrats. A liberal populist, Harkin sought the presidency in 1992 but dropped out and later backed Bill Clinton, who won the White House. 

Bill and Hillary Clinton addressed Democrats last year at Harkin's annual steak fry fundraiser, his final as a senator.