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Spenser Heaps, Deseret News
FILE - From left, Chris Herrod, Tanner Ainge and John Curtis appear in the Republican debate for the 3rd Congressional District primary race at the Utah Valley Convention Center in Provo on Friday, July 28, 2017. 

SALT LAKE CITY — Provo Mayor John Curtis continues to lead former state lawmaker Chris Herrod and political newcomer Tanner Ainge in the 3rd Congressional District GOP primary, according to a poll released Thursday.

The UtahPolicy.com poll comes as spending by so-called super political action committees supporting Herrod or Ainge approaches $1 million. Curtis is the main target of the advertising, which many have described as largely negative.

The poll found 31 percent of voters in the district either have already voted for Curtis or plan to in the Aug. 15 primary that most counties are conducting largely by mail.

Herrod followed in the poll with the support of 23 percent of the voters polled, and Ainge had 15 percent. But just over one-fourth of the voters — 26 percent — said they remain undecided in the three-way race for the GOP nomination.

Curtis, who has held a double-digit lead in past polls, said he is "thrilled each day that our strong Utah-based, Utah-principled and Utah-funded campaign continues to grow."

The two-term mayor said "dozens of volunteers are joining our campaign daily who believe that it's time we send some Utah values to Washington, D.C., and reject Washington, D.C., trying to buy this election."

Herrod campaign aide Ken Sumsion said Curtis' lead has fallen from previous polls because "Curtis is a Democrat, and Republicans are figuring that out." Curtis led the Utah County Democratic Party and ran for office as a Democrat years ago.

Sumsion said Herrod's internal polling shows him tied with Curtis, with Ainge trailing slightly.

Ainge's campaign spokesman, Peter Watkins, also said their internal polling tells a different story. Watkins said a robocall poll of 1,500 likely Republican voters put Ainge in the lead with 28 percent, Curtis at 25 percent, and Herrod at 22 percent.

"We feel confident our message of cutting taxes, cutting regulations and cutting spending in Washington, D.C., will resonate with voters," Watkins said.

The race is tightening since the last UtahPolicy.com poll, conducted in late June and early July. Then, 29 percent of self-identified GOP voters were behind Curtis; 12 percent behind Herrod; and 10 percent for Ainge; and nearly half, 49 percent, were undecided.

The latest poll was conducted Aug. 2, 3 and 8 by Dan Jones & Associates for the online political news source of 447 registered Utah voters in the 3rd District and has a margin of error of plus or minus 4.6 percent.

Only Republicans and unaffiliated voters who join the GOP at the polls on Election Day can vote in the primary. The poll's sample included 378 Republicans, 39 unaffiliated voters and seven Democrats.

Among just the Republicans polled, the race was closer, with Curtis backed by 29 percent of those surveyed, Herrod by 25 percent and Ainge by 16 percent, again with 26 percent still not sure who they prefer. There was no margin of error available for the smaller, GOP-only sample.

The primary will determine the Republican running in the November special election to fill the seat vacated June 30 by former Rep. Jason Chaffetz, now a Fox News contributor, through 2018.

Candidates already on the November ballot include Democrat Kathie Allen and the new United Utah Party's Jim Bennett. The 3rd District includes parts of Salt Lake and Utah counties, as well as Carbon, Emery, Grand, San Juan, and Wasatch counties.

On Thursday, the United Utah Party called for lawmakers to stop funding the Republican Party's closed primary elections that, unlike other political parties, prohibit nonmembers from voting.

The special election, set by Lt. Gov. Spencer Cox over protests by state lawmakers who said the executive branch doesn't have the authority, is expected to add some $1.5 million to the cost of holding already-planned municipal elections.

The Utah GOP chose to pay for last year's presidential preference caucus vote, forcing Democrats to do the same. Long lines for the caucus meetings resulted in a decline in voter turnout from past presidential primaries.

Utah GOP Chairman Rob Anderson said a federal court made it clear that Republicans could close their primary to nonparty members in striking down a provision calling for open primaries in a controversial law known as SB54.

The GOP is challenging the rest of the court's ruling upholding the law, which allows candidates to get on the primary ballot by gathering voter signatures instead of competing for delegate support at a party convention.

In the 3rd District primary, delegates selected Herrod as their preferred nominee over a number of candidates, including Curtis. But Curtis had also gathered voter signatures. So did Ainge, who did not compete at the convention.

Ainge's campaign released an automated call Thursday by former Alaska Gov. Sarah Palin, the GOP's vice presidential nominee in 2008. In it, Palin says, "We don't need any more career politicians in Washington. We need political outsiders — real patriots."