PORTLAND — As two former Oregon Democratic secretaries of state, Barbara Roberts and Phil Keisling don’t disagree on much. But a ballot measure that would upend the state’s current primary system and replace it with one open to any voter has the two at odds.

Proponents say the measure would give minor-party and unaffiliated voters a stronger voice in state politics.

But the very people the measure seeks to help are speaking against it. Nearly every political party in Oregon — the Independent and Working Families parties have taken no official stance — has panned Measure 65, which they worry could threaten their existence.

“We’re collateral damage in this whole idea,” said Seth Woolley, the Pacific Green Party’s candidate for secretary of state.

In Oregon now only voters who belong to a certain party may vote to determine that party’s general election candidate. That means that unaffiliated voters or minor-party voters have no say in which candidates the Republican and Democratic parties advance.

That would change if Measure 65 passes.

All Oregon voters would get a primary ballot that includes the name of every candidate for a given position regardless of self-identified party affiliation. Parties would be able to indicate their endorsement of a particular candidate on the ballots.

The two candidates who wrangle the most votes would face off in November but not as nominees of their particular party.

“I think it’s a very good idea,” said Keisling, one of the measure’s most ardent supporters. “I think it’s one that kind of speaks to the very ideals of our democratic society.”

Keisling has dedicated nearly four years to the change. He’s lobbied to push it through the Legislature twice and tried to get the measure on the 2006 ballot but fell short.

He says that the current system “locks out unaffiliated voters” and that the measure would “encourage as many people as possible to participate” and lead to more meaningful competition in Oregon, where many of the races are predetermined by county demographics.

Roberts, a former governor, doesn’t buy it.

“It sounds very good,” Roberts said. “It has feel-good language. But the truth is it doesn’t solve anything. It’s a solution looking for a problem.”

She points to Louisiana and Washington as examples of why Oregonians should reject the measure. In Louisiana, she said, the primary system has yielded low voter turnout and advanced polarizing candidates. In Washington, where a similar system was recently tested in the gubernatorial primary, voter participation dropped slightly.

“Why would we throw out a good system for one that has proven to be as flawed as this one has?” she asked.

Caught in the middle are the third-party voters and parties that the measure is supposed to help. Along with the state Democratic, Republican and Libertarian parties, the Pacific Green Party opposes the measure.

The Independent Party of Oregon has not taken a stance. Still, Linda Williams, the state party chair, said she has personal reservations.

“I would question whether, in practice, (the measure) accomplishes what I think is a really positive sentiment on the part of the backers,” Williams said. She worries about the measure for two reasons.

First, she said she doubts whether the system would really lead to a more moderate Oregon as Keisling has suggested. “What happens is you get a bunch of people in the middle who split the vote,” she said. That leaves candidates who can appeal to their party’s extreme base in a prime position.

Second, she says the measure could mean the end of some of Oregon’s smaller parties. Under the new legislation, in order to maintain status as a minor party, third parties would have to maintain a membership of about 10,000 voters. The Independent party can do that, but most others cannot, she and Woolley of the Green Party said.

“In Oregon, most of the minor parties aren’t able to get that many (members),” Woolley said. “They would essentially be eliminated in one election cycle by law.

“That just reinforces the two-party system.”