WASHINGTON, Aug. 11, 2015 - Donald Trump has taken the lead among Republican presidential candidates in Iowa as his GOP rivals prepare to make their pitches at the State Fair opening Thursday in Des Moines.

Two polls taken after the Aug. 6 Fox News debates show Trump favored by 19 percent and 17 percent respectively. Earlier polls showed Wisconsin Gov. Scott Walker leading in the state as recently as July, according to RealClear Politics.

But his opponents could take heart from a a Suffolk University poll released Tuesday that showed the debate hurt Trump. While he was leading by 17 percent to 12 percent for Walker among all GOP survey respondents in Iowa, the two men were tied at 14 percent among viewers of the debate. 

The debate watchers thought Florida Gov. Marco Rubio and retired neurosurgeon Ben Carson had the best performances. 

Trump may appear at the fair in Des Moines, according to his staff, but he isn’t scheduled to speak at The Des Moines Register-sponsored Soapbox as his rivals are. Former Arkansas Gov. Mike Huckabee will be the leadoff speaker on Thursday, and Florida Gov. Jeb Bush will appear on Friday.

Trump “is doing so well in the polls strictly because he’s very candid and because there are people who are sore, disgusted, with what goes on in Washington, D.C., and they see him as a person who has proven to the business world that he can take decisive action,” said Sen. Chuck Grassley, R-Iowa. 

Grassley will appear with at least three candidates at the fair, Rubio, Bush and Ohio Gov. John Kasich. He has offered to accompany any other candidates who ask, an aide said. 

The State Fair last month nixed Tump’s idea, floated in a British newspaper, of taking fair-going children for rides in his luxury helicopter.

Hillary Clinton’s Democratic rivals are also scheduled to appear at the Register Soapbox, including Vermont Sen. Bernie Sanders, who will speak on Saturday. 

Rubio was second in the Suffolk poll at 10 percent, followed by Carson at 9 percent. Texas Sen. Ted Cruz and businesswoman Carly Fiorina were tied at 7 percent. 

A poll by the Democratic-leaning firm Public Policy Polling put Trump at 19 percent, followed by Carson and Walker at 12 percent, Bush at 11, Florina at 10 percent and Cruz at 9 percent. Huckabee and Rubio each were at 6 percent. 

The PPP pollsters said Fiorina was the biggest winner in Iowa after the debates, even though she only appeared in a undercard event earlier in the day for seven candidates with the lowest poll ratings. She’s now viewed favorably by 56 percent of those polled, compared to 30 percent in April. Thirty-seven percent of Iowa Republicans watched that first debate, according to the Suffolk poll.

Clinton leads Sanders by 52 percent to 25 percent among Iowa Democrats, according to the PPP poll. 

The Suffolk poll was conducted Friday through Monday and based on live telephone interviews of Republican primary voter households that plan to participate in the caucuses. The margin of error is 4.4 percent. 

Public Policy Polling surveyed 619 usual Republican primary voters and 567 usual Democratic primary voters Friday through Sunday, 80 percent by phone and 20 percent over the internet. The margins of error are 3.9 percent and 4.1 percent for the Republicans and Democrats respectively. 

Editor's note: Philip Brasher and Sara Wyant will both be at the Iowa State Fair to cover the presidential candidates, visit with Iowa Ag leaders and sample the latest fair foods. Follow us on Twitter @agripulse for coverage from the event.

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