Iowa is one of the first states to pick who will run for president for each party, making it a key campaign target.

Its early caucus gives it disproportionate influence over the presidential nomination process despite its relatively small size, with about 1.8 million voters.

Parade march

With less than seven months to go before the primary and caucus season begins, candidates are keen to make an impression on what is a predominantly rural, small-town state.

Professor Cary Covington, a political scientist at the University of Iowa, said: "Iowa is the first state and so if you don't do well in Iowa, that puts you under a cloud for the whole way and makes it much harder to make progress in later states."

He believes Mrs Clinton has brought her husband into her campaign as early as she could, having built up a clear image of authority in her own right.

Mrs Clinton has been leading national polls but is trailing in Iowa to former North Carolina Senator John Edwards, who built up a strong base there when campaigning in the 2004 presidential race.

She maintains a lead over Barack Obama in the state but appears keen to regain ground lost to Mr Edwards.

Bill Clinton made his first major public appearance by her side at the Iowa State Fairgrounds in Des Moines on Monday, describing her as the candidate best qualified for the job.

The pair were due to campaign in Iowa City and Davenport on Tuesday and to take part in a 4 July parade in Clear Lake on Wednesday.