The race for Colorado governor is intensifying as the candidates chase ballots and trade attacks ahead of the June 26 primary.

Monday marked the first day county election officials could begin to count votes. The Colorado Secretary of State’s office reported that roughly 40,000 voters returned ballots in the first week, a fraction of the expected total this year with open contests for governor and other statewide posts on the Democratic and Republican tickets.

Eight years ago, when both parties last hosted open races for governor, turnout topped 32 percent with more than 774,000 votes counted, the office reported. This year, more people are eligible to cast ballots with the inclusion of unaffiliated voters – the largest bloc in Colorado at 1.2 million – for the first time.

The latest polls show clear front-runners for governor in both parties but significant numbers of undecided voters with two weeks to go.

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On the Republican side, two-term state Treasurer Walker Stapleton holds a double-digit edge, with 36 percent support among likely primary voters, according to a poll released Monday by Magellan Strategies, a Republican firm based in Colorado.

Victor Mitchell, a former state lawmaker and businessman, received 23 percent support with former Parker Mayor Greg Lopez at 10 percent and first-time candidate and businessman Doug Robinson at 4 percent.

About 27 percent in the poll – conducted Wednesday and Thursday – remain undecided. The survey’s margin of error is plus-or-minus 4 percentage points.

On the Democratic side, five-term U.S. Rep. Jared Polis sits in a similar position, with 31 percent support, according to a separate Magellan survey conducted May 30-31 of likely Democratic primary voters

Cary Kennedy, the former state treasurer, won 18 percent, with Mike Johnston, a former state senator, at 9 percent, and Lt. Gov. Donna Lynne at 3 percent. Still, 4 in 10 voters remained undecided in the survey, which had a margin of error of 4.4 percentage points.

“Based on those two polls, Jared and Walker appear to be the front-runners at this time, but neither race is over by any means,” said pollster David Flaherty.

The numbers evidently rattled Kennedy and Johnston. Both campaigns sought to reassure supporters about their strength in the race with emails that pointed to an unsubstantiated poll cited on a blog that showed them performing better than in the public poll.

“We are hearing about a lot of different polls … and we think this race is going to come down to the wire,” said Serena Woods, a Kennedy spokeswoman.

The limited polling in the race adds a wild-card factor. Flaherty said he hadn’t planned on polling the Democratic race, but a consultant who works with oil and gas companies that operate in Colorado paid for the survey.

To capture the decisive undecided voters, the candidates in both contests are turning negative. And Flaherty said he expects the attacks to accumulate in the final weeks.