File Photo: U.S. Democratic presidential candidate Senator Bernie Sanders (D-VT) talks to tribal members of the Sac and Fox Tribe of the Mississippi in Iowa/Meskwaki Nation during a campaign event at the Meskwaki Nation Settlement near Tama, Iowa, in this September 4, 2015 (Photo: Reuters)
Liberal US Senator Bernie Sanders has taken a slim lead over Democratic presidential frontrunner Hillary Clinton in Iowa, while billionaire Donald Trump extended his domination over the rest of the Republican field, polls showed Thursday.
A CNN/ORC poll finds Trump surging to 32 percent support nationally, becoming the first in the broad Republican field to gain to top 30 percent in the race.
The brash real estate mogul has gained eight points since August, and has nearly tripled his backing since shortly after launching his campaign in June.
The poll shows Ben Carson, a retired neurosurgeon, rising 10 points to take second place, with 19 percent.
Together, these two men who have never held public office are now backed by a majority of Republicans and Republican-leaning independents, according to CNN/ORC.
Florida ex-governor Jeb Bush is in third place with nine percent, a drop of four points since August.
Meanwhile in the Democratic race, Sanders nipped former secretary of state Clinton 41 percent to 40 percent in a Quinnipiac University Poll of likely Democratic caucus participants in Iowa, a vital US voting state because it holds the first contest to nominate the parties' flagbearer for the November 2016 election.
The come-from-behind result is the second in a month that shows the independent Sanders surpassing Clinton in a key state.
Last month, Sanders surged past her in New Hampshire, which holds the first primary after the Iowa caucuses, to snatch a 44-37 advantage, according to a Franklin Pierce University poll.
The results highlight a surging Sanders while Clinton, dogged by an email scandal, has seen her polling numbers slide in recent months.
Clinton was at 52 percent in Quinnipiac's July 2 Iowa poll, while Sanders was at 33 percent.
"Sanders has seized the momentum by offering a message more in line with disproportionately liberal primary and caucus voters," Quinnipiac polling assistant director Peter Brown said.
"He is the candidate of the Democratic left, against his own party's bosses and their prized presidential candidate, Secretary Hillary Clinton."
Clinton's favorability rating among Iowa Democrats remains high, at 76 percent to 20 percent, with voters saying 64 to 30 percent that she is honest and trustworthy, the poll showed.
That is far better than national polling, in which Clinton's favorability is underwater at 39-51, Quinnipiac reported late month.
Sanders's favorability rating is 78-6 percent, and 86 percent of voters say he is honest and trustworthy, compared to four percent who don't believe he is.