The Republican Party has “21 percent party ID and shrinking.”

Democrats are still celebrating over Sen. Arlen Specter’s party switch from Republican to Democrat — and are mocking the Republicans about it.

Their latest Web ad is a spoof of the television show

Survivor

, showing a parade of Republican leaders and asking, “Which one will win the battle for the heart and soul of the GOP?”

The ad was inspired by an op-ed that Republican moderate Sen. Olympia Snowe of Maine wrote for the

New York Times,

lamenting what she sees as the Republican Party’s hostile attitude to moderates.

“It is true that being a Republican moderate sometimes feels like being a cast member of

Survivor

— you are presented with multiple challenges, and you often get the distinct feeling that you’re no longer welcome in the tribe,” Snowe wrote. “But it is truly a dangerous signal that a Republican senator of nearly three decades no longer felt able to remain in the party.”

The DNC’s ad starts with that quote, then shows a parade of Republican leaders identified by their first names. There’s Mitt (Romney, former governor of Massachusetts); John B. (John Boehner, the House minority leader); Sarah (Palin, the governor of Alaska); Karl (Rove, political adviser to George W. Bush) and more. The ad concludes with Rush Limbaugh.

The video is really just an effort to mock Republican leaders. But one checkable fact stood out to us: “21 percent party ID and shrinking,” meaning that the GOP now has only 21 percent of the electorate, and that figure is declining.

To find out the facts, we have to turn to polls for how people cited their party because some states don’t require voters to declare an affiliation.

Two nonpartisan polling organizations, the Gallup Polls and Pew Research Center for People and the Press, released polling data on party identification close to the same time the DNC ad was released. Gallup found 28 percent identified as Republicans, while Pew counted it at 23 percent.

To broaden the picture, we turned to the Web site Pollster.com The site creates data models that aggregate many polls, with an eye toward more accurate results. The site lists an NBC poll at 20 percent and one from the Associated Press at 18 percent, but most are above the 21 percent cited by the Democrats. Using the range numbers, Pollster.com calculated an overall estimate of 26.4 percent.

So the Democrats are cherry-picking the low end.

The next question is whether Republican registrations are declining. The ad doesn’t give a time frame.

Both the Gallup and Pew polls show declines since roughly 2005. Gallup has recorded a decline in Republican Party identification from about 35 percent in 2005 to 28 percent in 2009. Pew showed a decline from 30 percent in 2004 to 23 percent in 2009.

During similar time frames, Democrats have seen small upticks or been flat. Pew showed independents increasing from 30 percent in 2004 to 36 percent in 2009.

So the DNC is right that the number of voters identifying themselves as Republicans has been declining. But 21 percent is from the low end of accepted estimates. So we rate their statement Mostly True.