The Republican presidential primaries have drawn an electorate that is much older and less ethnically-diverse than the rest of the country, exit polls in key states show, a pattern likely to continue as the GOP holds primaries on Tuesday in Alabama and Mississippi.

An analysis of the GOP primary electorate, as first detailed by National Journal’s Ron Brownstein, illustrates the increasing divide between Democrats, who are heavily reliant on support from blacks, Hispanics and voters under 40, and a GOP that is increasingly dominated by older and white voters.

For example, in the 2008 general election, 75 percent of voters in Arizona were white, 4 percent black, 16 percent Hispanic. But in the GOP primary last month, 89 percent of voters were white, 1 percent black, 8 percent Hispanic.

In the 2008 general election, there was about an equal number of voters between ages 18-29 and voters over 65 in Arizona. In the recent Republican primary, 23 percent of voters were over 65, while only 12 percent were between ages 18 and 29.

A similar pattern holds true in many of the key states that will determine who wins the fall election between President Obama and whoever is the Republican nominee. The Ohio electorate in the GOP primary last week was 96 percent white and a quarter over 65, compared to 83 percent white and 17 percent over 65 in the general election four years ago.

This divide is not surprising, as African-Americans have long voted Democratic, and Hispanics and voters under 30 also traditionally lean toward the Democratic Party. But it is already shaping this November’s election, even before the GOP primary is over.

Trying to keep together the coalition of voters they already have, the Republican candidates are marking little effort to court black or even Hispanic voters, who are a growing part of the electorate and potentially open to a strong appeal from the GOP.

They have expressed support for stringent immigration laws like the one in Arizona, which requires law enforcement officers to determine if people they arrest for other crimes are in the state illegally.

The candidates have been reluctant to detail the party’s ideas about making dramatic reforms to Medicare or Social Security, both of which might irritate the older voters who are now crucial to the GOP winning in November.

Conversely, when President Obama blames Republicans for the failure to pass an immigration reform bill that creates a pathway to citizenship for people here illegally, he is aiming at crucial bloc of his electoral coalition: Hispanics.

An older, whiter electorate turned out in 2010 than in 2008, helping Republicans make major gains in Congress. But many party strategists worry this approach won’t work in 2012, when more younger voters and minorities are likely to cast ballots.

And it creates a long-term challenge as well. As the Latino population continues to grow, Republicans will need to get a larger share of them to win elections in the future.

But in 2012, an electorate divided in this way is hardly ideal for Obama. His disapproval rate among white voters was at 51 percent in the latest NBC News/Wall Street Journal poll and 48 percent among voters over 65.

In states like Indiana and North Carolina, both of which were surprising wins for Obama in 2008, his growing disapproval among white and older voters may have outstripped demographic changes, particularly in North Carolina, which has a growing Hispanic population.

The Obama coalition would likely guarantee victory in 2020, but not with America’s demographics of today, where only about 30 percent of the electorate will be composed of racial minorities.

Follow Perry Bacon Jr. on Twitter at @perrybaconjr