Trump gets 33 percent support among the state's Republicans, doubling the nearest contender.
A new poll shows Clinton with 55 percent support to Sanders's 33 percent.
The two GOP candidates are locked in a virtual tie in the state, a new poll finds.
It's the second national poll in as many weeks to have Carson in the top spot.
Bobby Jindal scored 6 percent support in the survey from Public Policy Polling.
Webb has been floating the possibility of running as an independent.
The Monmouth University poll differs wildly from others showing a tighter contest.
Carson is tops in favorability: 84 percent of Iowa Republicans view him positively.
Carson wins 28 percent in Bloomberg poll to Trump's 19 percent.
Clinton has an 11-point lead in new Quinnipiac survey of Iowa.