PORTLAND, Ore. (AP) — Oregon's landmark new automatic voter registration system added nearly 52,000 voters in just four months this year, more than double what the state has normally seen for an entire year.

That sounds impressive, but there's a hitch. The so-called "motor voter" law — a first in the nation widely hailed as a way to boost voter participation — hasn't made it much easier to participate in Oregon's closed primary on Tuesday.

Unlike the November general election when all voters can participate, the presidential primary in Oregon and some other states is restricted only to voters who are registered as Republican or Democrat.

Under the new law, Oregonians 18 and up are automatically registered to vote while renewing or applying for a driver's license or state ID card, but they can't pick a party at that time. Instead, they're registered by default as nonaffiliated, and a few days later they can choose a party or opt out on a form sent by mail.

Most people, however, don't return their forms.

As a result, three-quarters of the motor voter registrants are unable to vote for a presidential candidate Tuesday.

Oregon Secretary of State Jeanne Atkins, whose agency is handling the law's rollout, defended the new system.

"I certainly won't argue that closed primary elections for brand new voters first time out is the way we're going to get the most participation out of them," she said. "The closed primary may not be the best example of when this will have a very big impact ... the real proof of the pudding will come in November."

California, Vermont and West Virginia recently adopted similar laws, while it's being considered in another two dozen or so other states.

That puts Oregon's implementation under a national microscope, said Jim Moore, a political science professor at Pacific University in Forest Grove, Oregon, west of Portland.

What's happened so far suggests the first wrinkle in fulfilling the law's promise of knocking down outdated barriers to voting, Moore said. It's also possible, he said, that some of the "motor voter" registrants are indifferent to politics, which was expected.

"They've in effect denied a choice to these newly registered, passively registered voters," he said. "It takes a bit just to actually get someone to register to vote, much less register for a party and then vote. And so it shows at this very lowest level, these passive registrants, they are less engaged than the rest of the electorate."

Aside from automatic registration, Oregon's presidential primary has seen unprecedented enthusiasm. A separate group of roughly 145,000 Oregonians switched to Democrat or Republican or signed up with those parties when they registered to vote.

For those who registered through motor voter, only 13,000 took that same initiative.

California, which is rolling out its motor-voter program next year, will allow people the option of registering to vote and also affiliate with a party on driver's license applications at the motor vehicle department. Party affiliation won't be an issue in Vermont, where primaries are open to all registered voters.

Atkins said it's worth discussing whether Oregon should change its approach.

"We're talking about a data-collection change at the department of motor vehicles that isn't part of their mission — I won't speak for that agency," she said. "I'm sure legislators will ask about that and we'll be happy to have the conversation."

Jonathan Brater, counsel for the Brennan Center for Justice's Democracy program at New York University School of Law, said the Oregon data show how critical party affiliations will be for other states to consider.

"As the law is continuing to be implemented, this is something that will continue to be important to keep an eye on," Brater said. "But I'd also say that, looking at the big picture ... as these additions accumulate over time and over multiple elections cycles, the civic culture in Oregon overall is only going to get better."