A recurring theme raised its head at last weekend's California Republican convention in Los Angeles: How to curtail the party's evaporation in the state. Fifteen years ago, 35 percent of registered voters were Republican. Today, 28 percent are.

Sen. Rand Paul (R-Ky.), there to rally the crowd (and lay more groundwork for 2016), even cracked a joke about the party's woes, according to the Los Angeles Times. "He said he'd heard that Republicans might have suffered a few recent losses in California, drawing chuckles from a crowd that's grown accustomed to getting clobbered," the paper reported. In a separate column, the Times explored the party's challenges.

What's interesting about California, though, is that voters aren't abandoning the party to become Democrats. They're largely abandoning the party for either third parties -- or for no one.

This isn't a new phenomenon; it's been written about as a national trend in the past. But what's happening in California differs dramatically from trends in other states.

Here's the trend there, according to official records from the California Secretary of State.

Compare that with the much smaller state of Wyoming, which has records going back to the 1960s.

In California, the Republican party is eroding and it's mostly among decline-to-states, the state's I-don't-want-to-choose option. (Though not entirely; more on that below.) In Wyoming, the Democratic party has withered -- and Republicans have seen the entire gain.

Now, look at New York.

New York also has a no-party option, but a decent chunk of the drop in Republican registration has been eaten up by the Democrats -- thanks in large part to a surge in 2008. Since 2004, Republican registration dropped 3.5 points; Democratic registration went up by 3.

And then there's Alaska, which has long had a non-partisan and then "undeclared" option. The state's registration been fairly static, with the slight dip in Democratic registration (which went up in 2008 despite the state's governor being on the national ballot), offset by Republican and "non-partisan" gains.

Four states, four rather different registration pictures. So what's happening in California?

The increase in decline-to-state is the big factor. Since 2004, 573,000 more people in California are Democrats -- about what you'd expect from a 1.1 million increase in voters. But the number of Republicans has dropped by over 700,000, while the number of DTS voters has gone up by 824,000.

The timing of those spikes is important. Registration surged during presidential years, which is normal. But from 2000 on, those decline-to-state voters could pick from any party that allowed them into their primary. In 2004, that was both major parties; in 2008 and 2012, the Democrats and a minor party. If you didn't want to ID as a Republican but wanted a choice in primary voting, decline-to-state was an option, albeit a limited one.

Another change might help explain the surge in third-party / other choices in 2012. That year, the state moved (back) to an open primary, advancing the top two candidates, regardless of party, to the November general. There was no penalty for signing up as a, say, Peace and Freedom Party member; you could still vote for statewide candidates as you wanted. (But not presidential ones. State parties kept those locked down.)

Changes in how the state votes are only a small part of the picture, of course. In general, shifting demographics and the national trend away from party identification and toward identifying as independents are certainly more significant components. In a sense, though, the California Republican party isn't doing as badly as, say, Democrats in Wyoming. At least California voters aren't leaving to become Democrats.