Breaking: Hillary's support fading in Iowa, now leads Bernie Sanders by just seven points

She’s the first choice of 37 percent of likely Democratic caucusgoers; he’s the pick for 30 percent, according to a new Des Moines Register/Bloomberg Politics Iowa Poll.

But Clinton has lost a third of her supporters since May, a trajectory that if sustained puts her at risk of losing again in Iowa, the initial crucible in the presidential nominating contest.

This is the first time Clinton, the former secretary of state and longtime presumptive front-runner, has dropped below the 50 percent mark in four polls conducted by the Register and Bloomberg Politics this year.

Poll results include Vice President Joe Biden as a choice, although he has not yet decided whether to join the race. Biden captures 14 percent, five months from the first-in-the-nation vote Feb. 1. Even without Biden in the mix, Clinton falls below a majority, at 43 percent.