Independent voters are suddenly the hottest commodity in American politics.

Independents propelled Sen. John McCain to victory in New Hampshire and limited his losses to Mitt Romney in Michigan. They helped build Sen. Barack Obama’s landslide victory in South Carolina. They will be up for grabs in Tuesday’s Democratic primary in California — but can’t vote for a Republican.

And if they are popular with the candidates now, by this fall they will be courted as never before.

Why? What’s going on?

Simple. In California and elsewhere, more and more voters are choosing to distance themselves from the Republicans and Democrats, and register with neither party. Amazingly, California has fewer Republicans and Democrats today than it did 20 years ago, despite an increase of millions of eligible voters. The greatest number of new voters, and plenty of old ones, have switched to “decline to state” — California’s official description for people who want out of the partisan system.

Between 1988 and 2004, the share of voters registered in no party increased from 9 percent to 18 percent while the Democrats’ share declined from 50 percent to 43 percent and the Republicans dropped from 39 percent to 35 percent. In that same period, the absolute number of Democrats and Republicans combined dropped by 800,000 while the number of independents grew by 1.7 million.

Independents tend to be younger, male and have more formal education than members of the major parties. As a group, they are more fiscally conservative than Democrats but more open to higher taxes than Republicans. On social issues, they are more liberal than Republicans but more conservative than Democrats. In recent years, they have tended to side with Democrats in statewide elections, but they have also been big supporters of Republican Gov. Arnold Schwarzenegger.

The trend appears to be part of a cultural shakeup driven by the information age. The Internet has broken down major institutions of all kinds in business, in media and entertainment while empowering individuals to exercise more control over their daily lives. The parties are going the same way.

“The reality is that independents are the future,” Bob Mulholland, a campaign adviser to the California Democratic Party, said last week in a joint discussion in which we participated on Capital Public Radio in Sacramento. “Young people don’t identify with one credit card. They don’t identify with one religious organization. They’re multiple choice, and that’s how they see life. A political party is just a vehicle to vote.”

Tom Del Beccaro, a vice chairman of the California Republican Party, believes the trend might be more cyclical, with independents waxing and waning depending on the effectiveness of the parties. But he agrees that the Republicans and the Democrats need to pay attention to the recent numbers.

“Both parties better address the issues these people are facing, or they will be deemed irrelevant,” he said.

Oddly enough, political conversation seems to be getting more and more partisan just as the independent wave is cresting. One reason is that, as independent-minded voters leave the parties, only the true believers remain, and thus primary campaigns tend to focus more on the issues that drive the most partisan voters. That trend is most evident in legislative races in California.

But in the presidential election, where the candidates still have to worry about the fall campaign, the four major candidates remaining are all vying for independent voters in one way or another.

In the Democratic race, Obama is openly appealing to independents and even Republicans with a call for consensus-building, though he has one of the most liberal voting records in the Senate. Sen. Hillary Rodham Clinton, on the other hand, has a history of pragmatism and is viewed with suspicion by Democratic liberals for her votes on the Iraq war and her past support for free trade and welfare reform. But she still has an image of partisanship that might be hurting her with independents.

On the Republican side, McCain has been a maverick in his party for a long time and relishes his independent reputation. That is one thing that led Schwarzenegger to endorse him last week. Former Massachusetts Gov. Romney, meanwhile, is running a more conservative-oriented campaign now but was once seen as a moderate. He famously crossed party lines to pass a health-care-reform plan in his home state.

But with independents shut out of the Republican primary in California, they will have to vote for a Democrat or no major candidate at all. Mulholland, the Democratic Party adviser, says his party’s open primary is a message to independents that they are welcome in the Democratic coalition. He says party officials hope that independents who vote for a Democrat on Tuesday will stick with the party in the fall.

Del Beccaro, the Republican official, says there is “no data” that suggests independents will stay with one party’s candidate all year.

“Long term, what matters is whether a party is relevant and productive in the minds of the voters,” he said.

Right now, in California and elsewhere, that description would seem to apply to neither party.

(Daniel Weintraub can be reached at dweintraub(at)