The already historic battle between Sens. Barack Obama and Hillary Rodham Clinton continues here on Tuesday and, for the first time in a long time, Pennsylvania may play a role in selecting a presidential nominee.

It won’t play the pivotal role. Delegate math makes it impossible for either candidate to secure the Democratic nomination by winning the Keystone State.

Obama holds the lead in delegates and the popular vote, but no matter how the remaining primaries go, he can’t secure the 2,025 delegates needed to win the party’s nod.

Clinton has gone from front-runner to second banana but she’s still holding the lead in Pennsylvania polls. She is hoping to perform well enough in the home stretch that she can convince superdelegates she’s got momentum, staying power and the expertise to defeat Republican Sen. John McCain.

It’s an exciting time to be a Democratic primary voter. So exciting that since January, 734 of Mercer County’s registered voters  — whether Republican, Independent or minor party — have switched parties to get in on the primary action.

With the new registrations, county Democrats outnumber Republicans by 7,420 voters. That’s more than 10 percent of the county’s primary-voting electorate.

While there’s also a four-way race for the 3rd Congressional District nomination, the presidential contest is clearly the main event and Director of Elections Jeff Greenburg is predicting 50 percent county-wide turnout for the primary.

“We expect to break a record as far as primaries go in Mercer County this year,” he said. “Clearly, we are going to surpass what we have seen in the last three presidential primaries.”

He expected turnout as high as 60 or 70 percent in the Shenango Valley and other Democrat-heavy precincts.

In Republican areas, it will likely be about 30 to 40 percent. “That will still be a lot more than what we’ve seen in the last presidential primaries,” Greenburg said.

Because that could mean 35,000 people voting on Tuesday, voters should allow themselves plenty of time if they intend to vote either right before or after normal work hours, especially in the Shenango Valley, Greenburg said.

Both Clinton and Obama have opened local campaign offices and support for the candidates is clear from the signs peppering roadsides from Stoneboro to the Shenango Valley. Oddly enough, they’re almost matched in some places by placards for Republican Ron Paul, who suspended his campaign after McCain secured the nomination.

Local Democrats have seen a few political near-celebrities make appearances here for Mrs. Clinton and Obama and partisans on both sides of the campaign were expected to turn out Saturday for a rally featuring Bill Clinton campaigning for his wife.

Demographically speaking, Mercer County’s Democrats should break for Mrs. Clinton. They’re older. They’re female and they come from the blue-collar tradition.