He campaigns for her in Iowa and other key states, spends hours each week on the phone with donors and elected officials and has helped forge policy ideas Clinton presents on the campaign trail.
In this way, Vilsack has made the quick and seemingly effortless transition from a one-time Clinton rival for the 2008 nomination to a go-to player in her campaign.
Aides acknowledge privately that Vilsack's work for the campaign has the look of a rehearsal for the role of running mate, should Clinton win the nomination.
But while he embarks on a busy summer for Clinton, no prospective No. 2 on a hypothetical Clinton ticket has a higher bar than Vilsack, whose first task is to ensure the senator's success in Iowa's leadoff caucuses.
Clinton and Vilsack in separate interviews with The Des Moines Register declined to discuss any collaboration beyond the nominating campaign, more than seven months until the caucuses launch it.
However, both acknowledge the depth of commitment by Vilsack, whom Clinton described as "one of the most effective and committed people in the country working for me right now."
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