The number of Californians who are registered to vote but are not affiliated with a political party has risen over the past four years, data released Tuesday show.

Despite the shift, registered Democrats still heavily outnumber Republicans in the most populous U.S. state, which recorded 17.6 million registered voters this year, up from 16.9 million in 2010.

Since 2010, the number of California voters who say they have no party preference has risen from about 3.4 million to about 4.1 million, the state said, from 20 percent of the electorate to 23 percent.

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One driver of the increase is the state’s shift in 2012 to a new system for nominating candidates that allows anyone to vote regardless of party, said Mark Baldassare, president of the Public Policy Institute of California.

Under the system, the top two vote-getters advance to the general election, regardless of party, making it less important for voters to declare an affiliation, Baldassare said.

“California is experiencing a dramatic increase in no party preference voters at a time when we have eliminated the partisan primaries,” he said. “While the share of no party preference voters has been rising for years, the state’s new voters have another reason no to affiliate with the Republican or Democratic parties.”

The change in registration also coincides with a drop in the number of Californians registered as Republicans in a state where both houses of the legislature and all statewide elected offices are dominated by Democrats.

Nearly 5.3 million Californians were registered as Republicans in September 2010, compared with just under 5 million in September 2014, the data showed. As a percentage of the electorate, Republicans dropped from 31 percent to 28 percent during the same period.

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The number of registered Democrats increased overall, from 7.5 million to 7.7 million, but as a percentage of all registered voters Democrats also dropped slightly, from 44.32 percent to 43.43 percent.

Among smaller parties, the number of voters affiliated with the American Independent party rose, from 399,000 to 474,000. The Peace and Freedom party picked up about 20,000 new adherents with 78,000 registrants in 2014, but the Libertarian party lost about 3,000, with 109,000 registered members in 2014.

(Reporting by Sharon Bernstein; Editing by Ken Wills and Cynthia Osterman)