THE INCREDIBLE SHRINKING PARTY (REDUX)…. The number of people who self-identify themselves as Republicans continues to shrink, as evidenced by four separate national polls released over the last five days. If this doesn’t scare party leaders, they’re not paying attention.

The Post/ABC poll found 21% of Americans identify themselves as Republicans. The NYT/CBS poll put the number at 20%. NBC/WSJ also put the GOP number at 20%.

The latest Pew Research Center study has a better take on the GOP’s standing, but only slightly.

Pennsylvania Sen. Arlen Specter’s switch to the Democratic Party on Tuesday highlights what is happening across the nation among Republicans — they are walking away from the GOP.

The latest survey from the Pew Research Center offers new data on the party’s diminishing ranks: just 23% of respondents identified themselves as Republicans, down from 25% in 2008 and from 30% in 2004.

Republicans have lost about a quarter of its base over the past five years.

Four polls, four results showing that only about a fifth of the population consider themselves Republicans. To put that in perspective, in 1992, Ross Perot and whatever it was his party was called got about 19% of the vote nationwide. Republicans are only slightly stronger now.

To be sure, this doesn’t necessarily translate into Democratic dominance. Fewer Americans identify themselves with the GOP, but they’re not rushing into the Democratic camp, either. Dems are doing well, and enjoy much stronger support than Republicans, but the majority party’s numbers aren’t dominating, at least not yet. For that matter, if the Obama administration’s policies fail to meet high hopes, it’s hardly a stretch to think Democrats’ numbers could see a decline at some point down the road.

That said, Republicans may not like it, but they should probably realize the scope of the mess they’re in if they hope to maintain the label of “major party” on a national scale.

Robert Farley had a good item on this last night: “[P]olitical parties do die. They don’t die often, but even in the United States they sometimes go belly up. I think that the Republican Party has become stuck in an ideological and demographic trap of its own making, and I’m not sure that it understands the seriousness of the situation.”

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.