Supporters of Republican presidential candidate and former Massachusetts Governor Mitt Romney look on during a campaign rally at Flagler College on August 13, 2012 in St Augustine, Florida. (Photo by Justin Sullivan/Getty Images)

Mitt Romney’s decision to tap Paul Ryan, instead of a woman or minority, as his running mate will leave in place the huge demographic gap that has defined the 2012 campaign cycle: Obama leading among blacks, Hispanics, women and voters under 30, while Romney is ahead among older and white voters.

In choosing Ryan, Romney opted against a direct appeal to the women’s vote, unlike John McCain, who chose Sarah Palin four years ago. He is also not making a sharp pitch to Hispanics by picking a Latino candidate, such as Florida Sen. Marco Rubio. And he rejected the urgings of some Republicans to choose former Secretary of State Condoleezza Rice, who had the potential to help Romney with minorities, women and moderate voters, and reshape the contest.

Republicans note Ryan, at age 42, does represent a new generation for the GOP and will help appeal to young voters better than the 65-year-old Romney. Democrats in turn argue that the congressman’s controversial plan to remake Medicare will pull elderly voters away from Romney.

But those differences are likely to be at the margins. Ryan’s core appeal is to conservatives who like his commitment to budget cuts and less government intervention in health care and the economy. And he is expected to court working-class voters in states like Ohio and Wisconsin who have been resistant to both Romney and Obama.

In fact, the selection of Ryan makes this election even more one in which both sides are aggressively trying to turn out their bases. Ryan helps ensure Tea Party Republicans, many of whom are lukewarm about Romney, are motivated not only to vote, but volunteer and raise money. Democrats will highlight how his budget cuts would affect women and minorities, two of their most importing voting blocs.

Romney’s decision to pick Ryan will leave him heavily reliant on white voters who are either traditional Republicans or those who voted for Obama four years ago but have soured on him since. Ryan doesn’t come with any of the risks of Rubio, who has a more controversial record in Florida, or Rice, who could have annoyed social conservatives because she has said in the past she supports abortion rights.

But he has also offers few rewards, particularly among Latino voters, whose lack of support for Republicans could hurt Romney in Florida, Colorado and Nevada, or women, who of course are the majority of the electorate.

The pick will keep voters divided among increasingly familiar lines. Obama leads by more than 80 points among blacks, 40 among Hispanics, and has double digit advantages with women and voters under 30 in most polls. Romney is keeping the race close with sizable advantages among men, white evangelical Christians, and white voters overall.

Follow Perry Bacon Jr. on Twitter at @perrybaconjr