PIERRE, S.D. (AP) — A national nonprofit has pledged $140,000 to help supporters of a constitutional amendment that would move South Dakota to an open primary system for many races, the nonprofit's spokesman said Tuesday.

New York-based Open Primaries is supporting the amendment campaign's signature-gathering efforts, spokesman Jeremy Gruber said. The proposed amendment would have the top two finishers in a primary advance to the general election regardless of party.

Backers of the amendment hope to start gathering signatures around Sept. 1, campaign chairman Joe Kirby of Sioux Falls said. They must submit nearly 28,000 valid signatures to the secretary of state by November 2017 for the amendment to appear on the 2018 ballot.

Open Primaries South Dakota Treasurer De Knudson said the group badly needed financial assistance for the "gargantuan" task of collecting signatures.

"Volunteers will be working extremely hard throughout the state to help make this happen, but it will be necessary to have some professional help also," Knudson said.

The proposal would apply to primaries for county offices, Legislature, governor and U.S. House and Senate. For example, in a gubernatorial race under the plan, there would be an open primary for all candidates in which the top two vote-getters would advance to the general election.

Supporters say the measure would be fairer because many elections now are effectively decided in partisan primaries closed to independent voters. In South Dakota, the Democratic primary is open to independents, while the Republican primary is closed.

"The basic pitch is that open primaries let all voters vote," Kirby said.

There are roughly 122,000 independents, 165,000 Democrats and 250,000 Republicans in South Dakota, according to secretary of state's office voter registration totals.

The 2018 push comes after a similar amendment failed at the polls last year. Kirby said supporters have listened to voters and dropped a provision that would have removed party labels from ballots.

He said he thinks the new campaign's budget will be "much smaller" than the previous attempt, saying that supporters plan to rely more on letters to the editor, town hall meetings and networking around the state rather than expensive advertising.

Open Primaries put more than $1 million toward passing the 2016 constitutional amendment, which received 44.5 percent support. Gruber said the group hasn't committed to supporting anything beyond signature-gathering efforts for the new proposal.

"They asked us to work with them, and we're excited to do so," Gruber said.

Democratic Chairwoman Ann Tornberg said her party hasn't taken an official position on the plan, and Republican Party Chairman Dan Lederman didn't immediately return a telephone message requesting comment from The AP.