Republicans and Democrats will announce a plan for a Colorado presidential primary Thursday that would allow the state’s unaffiliated voters to participate.

More than one-third of Colorado voters — the largest bloc — are not affiliated with a party. The legislative proposal would allow them to choose which party’s primary they would wish to participate in.

Thirty days after the primary the registration would expire, said Rep. Dominick Moreno, a Democrat from Commerce City, one of the authors of the bill.

The measure would apply only to the presidential race. All other party caucus operations, including selecting delegates in the nomination process, would remain the same, he said.

“We’re going to make it as easy as possible for unaffiliated voters to actually vote in the primary,” Moreno said. “That would include allowing the unaffiliated voter to request which ballot they want, and it would result in them being temporarily affiliated with the party for the period of the primary.”

“… At the end of the day we want as many people to participate in the primary as possible.”

Cost is expected to be a focus of debate. Parties pay for the caucuses, but state taxpayers would be on the hook for a primary, with estimates ranging from $2 million to $7 million.

As much as two-thirds of that cost would be borne by counties, with the state picking up the rest.

Republicans and Democrats in Colorado were roiled by claims that this year’s caucuses did not represent a majority rule.

Supporters of Donald Trump, who received none of Colorado’s 34 delegates this year, protested at the state Capitol last Friday, demanding a vote. Republicans canceled the presidential preference poll at their March 1 caucus.

And in New York this, supporters of Bernie Sanders railed against the state’s exclusion of unaffiliated voters.

Democratic voters complained of a chaotic caucus this year, as party volunteers tried to manage a large turnout.

Unaffiliated voters in Colorado this year had to register with a party almost two months in advance to participate in the caucus, then change their voter registration afterward.

Allowing unaffiliated voters is a major concession from Republicans who have blocked previous attempts to do so, including a bill that was killed in the 2015 session.

Ballot initiatives to restore a primary in Colorado have been floated.

“We’ve all heard loud and clear in Colorado that folks want their voices heard in a primary,” said Senate President Bill Cadman, a Republican from Colorado Springs. “I’d like to have my voice heard in a primary in Colorado. There are different sentiments about what that looks like. Some folks want to get rid of the caucus system, some folks want to open up the primaries (to unaffiliated voters). With some concerns with what could happen outside this building with the initiative process, I’d like to be a part of providing a solution that I think is appropriate for this state.”

Colorado had a caucus from at least 1912 to 1992, the switched to primaries. The state reverted to a caucus system in 2002. .

Joey Bunch: 303-954-1174, jbunch@denverpost.com or @joeybunch