Walker Stapleton is leading Colorado’s four-way Republican primary for governor, but businessman Victor Mitchell’s late ad blitz could position him to close the gap in the final weeks of the campaign, according to a poll released Monday by GOP firm Magellan Strategies and made available early to Colorado Politics.

The survey of nearly 600 likely voters in the Republican primary found Stapleton, Colorado’s two-term state treasurer, leading with 36 percent support among those surveyed, 13 percentage points ahead of Mitchell’s 23 percent.

PRIMARY PREVIEW: Our guide to the June primary election is in this week’s Colorado Politics print edition.

Restaurant owner Greg Lopez, a former Small Business Administration official, has support from 10 percent of those surveyed, while retired investment banker Doug Robinson, a nephew of Mitt Romney, has 4 percent.

With 27 percent of likely primary voters still undecided and Mitchell spending heavily on attack ads hammering Stapleton, pollster David Flaherty described the contest as “far from over” in a memo summarizing the survey’s results.

“A major factor that will decide this race is if Victor Mitchell’s attack ads are effective moving voters away from Walker Stapleton,” Flaherty said.

A Magellan poll conducted in late March, before the final GOP primary ballot was set, also found Stapleton ahead but had Mitchell in low single digits. Since then, the former state representative has spent millions on a TV ad campaign introducing himself to voters.

Just as mail ballots went out to voters last week, Mitchell’s ads turned negative.

In one ad, tennis-playing cartoon characters drive home the point that Stapleton is related to the Bush political dynasty – both former Bush presidents are Stapleton’s cousins – while another Mitchell ad tears into Stapleton for repeating a false claim about his support for President Donald Trump’s tax package.

Flaherty said that Stapleton’s broad popularity among older voters could stymie Mitchell’s chances of turning the tide.

“For Victor Mitchell to be in a position to win, his negative ads have to work with only two weeks to Election Day,” Flaherty said. “We are somewhat surprised (about) the timing of Victor Mitchell’s attack ads coming so late in the election.”

According to the survey, 51 percent of likely voters have a favorable opinion of Stapleton, and just 15 percent view him unfavorably.

Mitchell’s image isn’t far behind, with 39 percent viewing him favorably and the same 15 percent viewing him unfavorably, but Flaherty suggested that Mitchell is facing a “significant challenge … driving voters away from Walker Stapleton,” particularly among voters 65 and older.

Those voters, accounting for half of those who plan to vote in the GOP primary, hold favorable impressions of Stapleton by a wider margin than the electorate in general. And members of that group tend to turn in their ballots early, Flaherty noted, reducing the potential impact of advertising in the remaining weeks.

“With two weeks before (the primary voting deadline), is the Republican nomination for governor a done deal?” Flaherty wrote. “Based on the findings of this survey, absolutely not. However, it is hard to argue that Walker Stapleton does not have the inside track and that Victor Mitchell has a more difficult path to victory.”

The survey was conducted by live interviewers and automated calling devices using cell phones and landlines on June 6 and 7 – as county clerks were mailing ballots to voters.

For the first time, unaffiliated voters can cast ballots in either the Republican or Democratic primary without having to change their registration.

According to the poll, just 14 percent of the likely voters are registered unaffiliated. That’s a slightly lower share of the primary vote than the 17 percent Magellan predicted in the Democratic primary.

Results in the GOP survey were weighted to reflect turnout demographics of past primary elections in Colorado. It has a margin of error of plus-or-minus 4.02 percent.

The survey was paid for by Magellan Strategies, said Flaherty, who noted that the Louisville-based firm hasn’t been hired by any of the candidates or committees working in the gubernatorial primary races.