The cited poll must be acknowledged as not particularly useful in measuring the wide variety of high attention social issues from which Americans define themselves as C, M or L.
If we just use those three words, then of course we're going to see more C and M because quite frankly there's been a rather successful framing of the word "liberal" during the past 20 years as being something less friendly than C or M.
Though in fact, none of the three - C, M or L - are any more friendly, less friendly, mean, selfish, yada yada as any of the others if given a big enough sample from each group.
What's clear to me is that not only are those who have entered the voting ranks during the past 12 years more comfortable with self-defining as "Liberal", the overall attitudes of the 18-45 age range are indisputably more comfortable with tolerating and/or endorsing socially liberal attitudes and public policies.
Standing a firm ground for most anyone 30 and above is "conservative financially" and I don't see that changing much in coming years.
Thus, on financial and business matters, I perceive that more Americans will remain staunchly center/right in next 10 to 20 years. And on social matters, there will continue to be a plodding, but nonetheless relentless tilt towards more tolerant and liberal mindsets.
I think that in the coming couple of decades, smart thinking Americans of all stripes will realize that it's far more urgent to keep our personal and our public finances in order than it is to burn needless energy trying to police adult consensual behaviors.
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As for the lead topic of Specter "switching to Dem", that's a "duh!".....He's just one of the two or three guys who each election cycle jump parties in hopes of securing their grip on whatever office they hold. It's hardly an indicator of overall national trends. Rather, it's an isolated example of someone trying to get an edge in what is a pretty high-stakes career.