Mel Bingaman couldn't be more excited to cast her vote for Donald Trump – "a real man of the people," she says, when Pennsylvania and four other states hold primary elections on Tuesday.

But a reminder here at a raucous Trump rally to also notch her ballot for delegates who have committed to vote for her candidate at the Republican convention this summer left her baffled.

"I know I'm going to vote for Mr. Trump," the Selinsgrove nurses aide said, emerging from the rally with a yard sign and a campaign button. "But I don't know about this delegate business. Who am I supposed to vote for?"

With the possibility that the Republican presidential nomination won't be wrapped up until the July convention, Pennsylvania and its unique method of picking delegates who will cast the actual votes at the convention could play a critical role and serve as a test case for Trump's efforts to professionalize his campaign and nab delegates, not just the popular vote.

The top Republican vote-getter in Tuesday's Pennsylvania primary wins just 17 statewide delegates. Three more delegates are allocated to state party leaders.