With three weeks to go until the Republican primary, Provo Mayor John Curtis has tightened his grip on the lead in the special election to replace former Rep. Jason Chaffetz.

The latest Salt Lake Tribune-Hinckley Institute of Politics poll shows Curtis trouncing his competitors by a more than 2-to-1 margin.

"When the voters go into the ballot box, you want them to be able to look at the names and recognize one of them," said Jason Perry, the institute's director. "That is something that John Curtis has spent a lot of time on, and I think that's reflected in this poll."

With 37 percent of Republican respondents backing his bid, Curtis steamrolls the nearest GOP candidate, Utah County businessman Tanner Ainge, who came in at 17 percent. Former state Rep. Chris Herrod collected 14 percent of the vote in the mock primary.

The 3rd District poll included responses from 234 Republicans who are registered to vote. Only registered Republicans can vote in the GOP primary. The poll was conducted by Dan Jones & Associates from July 18 to July 21, and it has a margin of error of plus or minus 7.5 percentage points.

While the results maintain Curtis' upper hand from an earlier Utah Policy poll, when he fetched 29 percent of the vote, Ainge has now worked his way to second place, flipping Herrod into last. Herrod doesn't completely buy into the findings, saying his internal campaign polls have him "much closer than that, within striking distance of John."

Scott Sommerdorf | The Salt Lake Tribune Chris Herrod, and his wife Alia, right, react as the final voting count is announced. Herrod was chosen at the Republican convention to run on the GOP ticket to fill Jason Chaffetz's vacant seat in Congress. The convention was held at Timpview High School, Saturday, June 17, 2017.

Still, Perry said, "there are opportunities in these numbers for all of the candidates." In the new poll, 41 percent of all voters remain undecided, including 32 percent of Republicans — what Perry calls "a huge number" — down from 49 percent. As the primary looms closer, he said, that group will continue to shrink.

So far, Curtis said it seems those individuals are leaning his way.

"People have watched Provo and know what's happened here," he said.

He's not wrong. Of the GOP candidates, Curtis has the highest favorability rating among Republicans, 50 percent, and the lowest fraction of GOP respondents who report having never heard of him, 21 percent. Perry credits that to Curtis' time managing Utah's third largest city.

"He is a sitting mayor that has been very active," Perry said, later adding: "Herrod is a former legislator, and people may remember him from his time there. But he has not been in an elected office for some time."

Thirty percent gave Herrod a very or somewhat favorable score, while 27 percent said they did not know of him. That didn't surprise Herrod.

"I could go to the grocery store at any time and nobody would know who I am," he said. "That's kind of a reminder that even though you've been involved in state politics, most people aren't paying attention."

Ainge, on the other hand, gains some name recognition from his father, Danny Ainge, general manager of the Boston Celtics.

Leah Hogsten | The Salt Lake Tribune Tanner Ainge addresses the crowd. Americans for Prosperity-Utah hosted a 3rd congressional district candidate debate to replace Rep. Jason Chaffetz's vacancy. The candidates, Provo Mayor John Curtis, former state Rep. Chris Herrod, investment adviser Tanner Ainge, Libertarian Joe Buchman and Independent American Jason Christensen fielded questions at the Covey Center for the Arts in Provo. The primary will be held Aug. 15.

With a slight edge over Herrod, Ainge has a 34 percent favorability rating with 26 percent of Republicans respondents not having heard of him.

The important part, Perry said, is that the candidates don't bolster any unfavorable ratings, which are all relatively low for now. Plenty of eyes are on the special election, he said, with seven out of 10 reporting more than a passing interest in the race.

The winner of the primary will face Democrat Kathie Allen in the Nov. 7 general election. While she's amassed nearly $700,000 in donations, she is the least known in the overwhelmingly red district. Some 54 percent of Republicans reported never hearing about her, neither has 31 percent of Democrats.

Curtis suggests his poll numbers — and endorsement from Utah Gov. Gary Herbert — make him the "best positioned for a race with Allen." Ainge isn't so sure. He picked at a common criticism of Curtis: that the mayor was a registered Democrat for a short time about 20 years ago.

"Voters have a chance to learn and research now … they will not trust a former Democrat with the Republican nomination," Ainge said. "They want someone from outside politics to shake things up."

The two, Ainge and Curtis, both got on the primary ballot by collecting signatures, which caused some uproar among GOP delegates who selected Herrod as their nominee. Yet most conservative residents in the district, 68 percent, support the alternate route to the ballot that bypasses the traditional convention.

Perry calls that "extremely significant."

"I think that is at the heart of the division within the Republican Party."