Republican presidential candidate Donald Trump leads the 2016 GOP field in Iowa and Sen. Bernard Sanders has a slight edge over Democratic presidential rival Hillary Clinton in the state, according to polling released Monday as voters prepare for the Iowa caucuses.

Both Mr. Trump and Mr. Sanders had big leads among first-time caucus-goers, meaning the two candidates could need to turn out people new to the political process if they hope to win on Monday.

On the Republican side, Mr. Trump was at 31 percent support, followed by Sen. Ted Cruz of Texas at 24 percent, Sen. Marco Rubio at 17 percent and retired neurosurgeon Ben Carson at 8 percent in the Quinnipiac University poll. More than a quarter of voters who named a candidate said they still might change their mind.

Mr. Trump led Mr. Cruz by an 18-point, 40 percent to 22 percent, margin among first-time caucus-goers, while Republicans who have attended prior caucuses support Mr. Cruz by a 1-point margin, 26 percent to 25 percent.

Mr. Sanders, meanwhile, had a 3-point edge over Mrs. Clinton, 49 percent to 46 percent, which included a 62 percent to 35 percent advantage among first-time caucus-goers.

Mrs. Clinton, meanwhile, had an 11-point, 52 percent to 41 percent, edge among voters who have participated in previous caucuses.

“The size of the turnout tonight will likely be the key factor, especially on the Democratic side,” said Peter A. Brown, assistant director of the Quinnipiac University Poll. “High turnouts with lots of new caucus participants likely would mean a good night for Sen. Bernie Sanders, and for Donald Trump.”

The polling was conducted from Jan. 25-31; the final GOP presidential debate before the Iowa caucuses was held Jan. 28, although Mr. Trump did not participate.

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