Washington ? For all the attention generated by Barack Obama’s candidacy, the share of eligible voters who actually cast ballots in November declined for the first time in a dozen years. The reason: Older whites with little interest in backing either Barack Obama or John McCain stayed home.

Census figures released Monday show about 63.6 percent of all U.S. citizens ages 18 and older, or 131.1 million people, voted last November.

Although that represented an increase of 5 million voters — virtually all of them minorities — the turnout relative to the population of eligible voters was a decrease from 63.8 percent in 2004.

Ohio and Pennsylvania were among those showing declines in white voters, helping Obama carry those battleground states.

“While the significance of minority votes for Obama is clearly key, it cannot be overlooked that reduced white support for a Republican candidate allowed minorities to tip the balance in many slow-growing ‘purple’ states,” said William H. Frey, a demographer for the Brookings Institution, referring to key battleground states that don’t notably tilt Democrat or Republican.

“The question I would ask is if a continuing stagnating economy could change that,” he said.

According to census data, 66 percent of whites voted last November, down 1 percentage point from 2004. Blacks increased their turnout by 5 percentage points to 65 percent, nearly matching whites. Hispanics improved turnout by 3 percentage points, and Asians by 3.5 percentage points, each reaching a turnout of nearly 50 percent. In all, minorities made up nearly 1 in 4 voters in 2008, the most diverse electorate ever.

By age, voters 18-to-24 were the only group to show a statistically significant increase in turnout, with 49 percent casting ballots, compared with 47 percent in 2004.