To be sure, turnout in presidential primaries is not completely fueled by the top of the ticket. Competitive state and local races can also have an impact. McDonald also cautioned that linking turnout to the presumptive presidential nominee doesn't take into account that some voters, albeit a minority, are choosing one of his rivals.
Having said that, turnout in presidential primaries in Pennsylvania, one of the most important battlegrounds in the country, hasn't varied tremendously from year to year. It peaked slightly in the 2008 Republican primary to 26 percent even though John McCain had locked down the nomination more than two months earlier.
In the 2004 Democratic primary, held weeks after John Kerry claimed the nomination, turnout was 21 percent. In 2000, when both Republican George W. Bush and Democrat Al Gore claimed their nominations early in the primary season, turnout in Pennsylvania was 20 percent and 19 percent, respectively.
A five-way Republican primary for the U.S. Senate this year could help drive voters to the polls in Pennsylvania. Still, it's fair to say that turnout much below these recent benchmarks, or much higher, would indicate something about the level of enthusiasm for Romney.
Not surprisingly, both the Obama and Romney campaigns declined to reveal their expectations for Tuesday's turnout and what it might say about the race.
"You can't completely dismiss the presidential primaries at this point,'' McDonald said. "I'm certain Romney is going to be the nominee, but I think his campaign has an incentive to be active on Tuesday. It's sort of a dress rehearsal for the general election.''
A better gauge of the enthusiasm surrounding a candidate, McDonald said, is county-by-county turnout. He studied voting patterns in two other big states that voted in January and February, Florida and Michigan, and found that turnout in counties that favored Romney was down compared to 2008, while turnout was up in counties that favored his rivals. "They're just not that into you, Mitt,'' McDonald wrote after Florida's vote.
In an analysis of the first 13 primaries, the Bipartisan Policy Center and the Center for the Study of the American Electorate pegged Republican turnout at 11.5 percent, compared with 13.2 percent in those same states in 2008 and 12.2 percent in 2000.
In a more recent sign of limited excitement about Romney, only one out of three Republican voters in a CBS/New York Times poll earlier this month said they would "enthusiastically'' support Romney. Forty percent said they back him with reservations and 18 percent said they support him only because he's the nominee.
But voter enthusiasm doesn't necessarily translate into turnout. A vote from someone who's not crazy about Romney counts just as much as a vote from an avid supporter. What's more, hostility toward President Obama may push many Republicans to the polls in November. The most recent CNN/ORC International survey found that more than six in 10 Romney supporters say their choice will mostly be a vote against Obama, while three-quarters of Obama's supporters say they are backing the president, not voting against Romney.