INDIANAPOLIS (AP) — What voters might not know as they cast their ballots in the Indiana primary is that not all ballots for a Democratic presidential candidate will count equally.

Every vote for Barack Obama or Hillary Rodham Clinton should be part of the overall tally on May 6. But when dividing the 72 pledged delegates at stake, votes cast in the state's four Republican-held congressional districts will go toward a slightly smaller share.

The expected two-week dash for votes across Indiana might translate to only a slight advantage for one of the candidates after the proportional awarding of delegates.

Nearly two-thirds of Indiana's delegates will be divided among the nine congressional districts with the rest allocated based on the statewide vote.

The five districts with Democrats in Congress are allocated more delegates than the four with Republican representatives. The Democratic-held districts will each determine six delegates, while GOP-held districts have four or five.

That distribution, like in other states, is based on Democratic voting strength in the most recent presidential and gubernatorial elections, said Dan Parker, state Democratic chairman.

For instance, the largest block of national convention delegates is 16 at-large positions determined by the statewide results. In order for Obama or Clinton to gain just a 9-7 delegate advantage, one would need to win the state with about 55 percent, Parker said.

And in the congressional districts with six delegates, a candidate needs a 58 percent majority to win a fourth delegate.

Those national delegates are not actually elected in the primary, either. They will be selected at the state party convention in June from slates approved by the Clinton and Obama campaigns, said Parker.

"They have to approve of the names that are pledged because they have to make sure that the folks are bona fide supporters," Parker said. "The ways they will be able to tell: Did the person work for us? Did the person endorse us? Did the person contribute to us? Are they really a supporter?"

Indiana's delegation to the Democratic National Convention in Denver will also include 13 superdelegates — party leaders and elected officials who are free to vote as they wish.

Five of those superdelegates, including Sen. Evan Bayh and Parker, have endorsed Clinton, while three are supporting Obama. Four of Indiana's five Democratic congressmen, who are all superdelegates, have yet to pick.

Parker said he expected it would take at least two weeks for state election officials to certify the primary votes and for the state party to determine the delegate breakdown.