Among the many good deeds done by the Pew Research Center is a handy comparison of its June polls, conducted during each of the last three presidential election cycles. Holding other variations constant by using the same pollster and the same methodology at equivalent points in the cycle, and by interviewing large samples, the surveys paint a fascinating portrait of changing electoral demography.

Differences begin with the horse-races themselves. Al Gore led George Bush by one point — coincidentally, almost exactly his eventual popular-vote margin — while Bush had a two-point advantage over John Kerry — half a point off his November showing. Barack Obama beats John McCain by eight points in Pew’s most recent iteration.

Beneath the surface, there are some significant changes, as well as striking continuities in the candidates’ bases of support.

Perhaps the most dramatic reshuffling has been generational. Obama is continuing the trend John Kerry began toward a breakout performance among younger voters. Gore was actually losing 18-to-29-year-olds in June 2000 by five points. By contrast, John Kerry was winning them by 11, while Obama has nearly doubled Kerry’s advantage, leading the youngest cohort by 20 points.

Obama has also expanded Democratic support among the next youngest segment (30-to-49-year-olds). Gore was down one point with these voters and Kerry behind by eight; but Obama has leapt to a 13-point lead in this cohort.

However, the generational reshuffling carries with it some disquieting signs as well. Having expanded his totals among younger voters, Obama is falling short among seniors. At this stage in his race, Gore led by three among those over 65, while Kerry enjoyed an eight-point advantage. Obama, though, is running seven points behind McCain among older voters.

The causes are difficult to discern at this point. Perhaps it is another manifestation of the identity politics that surfaced clearly during the primaries — younger voters can certainly identify more directly with Obama, while older voters see a clearer reflection of themselves in McCain. Or perhaps it is further evidence of the cultural polarization of our politics, as age remains one of the central dividing lines between culturally progressive and culturally conservative voters.

Culture’s centrality is reinforced by examining religious influences on voting behavior. Despite all the talk about a weakening bond between white evangelicals and the Republican Party, in Pew’s data Obama’s deficit (36 points) among these voters is nearly identical to Gore’s (37 points). Obama does outperform Kerry in this segment (-36 vs. -43), but with Obama doing far better than Kerry among whites overall (-8 for Obama, -15 for Kerry), in relative terms, white evangelicals are no less Republican than in the past.

More striking evidence of the cultural alignment comes in the dramatic changes affecting secular voters. Those who claimed no religious affiliation supported Gore by a six-point margin, Kerry by 36, and now give Obama a yawning 43-point advantage.

Thus, the gap between the most secular voters and those who at least claim a religious affiliation is larger than ever.

Meanwhile, despite the historic nature of this election, the racial gaps are quite similar to those in earlier cycles.

Obama is getting slightly more African-American support (90 percent) than did Gore (85 percent) or Kerry (87 percent), offering Obama relatively less room to grow with this base constituency. But the black Democrat is doing better with whites than either of his Democratic predecessors — he trails by just eight, compared to a nine-point deficit for Gore and 15 for Kerry.

So while the odds continue to favor Obama strongly, his coalition may prove to be significantly different than that assembled by other Democrats.

Mellman is president of The Mellman Group and has worked for Democratic candidates and causes since 1982, including Sen. John Kerry (D-Mass.) in 2004.