COLUMBUS, Ohio >> Republican Sen. Rob Portman faces a tough re-election in his swing state of Ohio, and it just got more complicated with the partisan fight over whether President Barack Obama should fill a Supreme Court vacancy in his last year in office.

Two days after Justice Antonin Scalia’s death, Portman fell in line behind his Republican leader, Sen. Mitch McConnell, and said the American people in November’s election should “weigh in on who should make a lifetime appointment that could reshape the Supreme Court for generations.” Portman’s leading Democratic challenger, former Gov. Ted Strickland, pounced.

In a fundraising appeal on Thursday, Strickland said, “Senator Rob Portman is flat-out refusing to do his job.”

Democrats have kept up a barrage of criticism, calling on Portman to show independence and break from the party line. P.G. Sittenfeld, another Democrat seeking Portman’s seat, said the Republican was simply wrong on the issue. Major state newspapers in Toledo and Columbus also have called on Portman to act on any nominee sent to the Senate by a twice-elected president who still has 11 months left in his term.

Portman stuck with his opposition to Obama filling the seat this week while in Cincinnati for an event featuring tech start-ups and app developers.

“I think you want to do the right thing by the country,” Portman said in an interview. “And to me, to have us embroiled in what will be a very controversial nomination here in his last year is not as good as allowing the people to decide.”

He said the vacancy and discussion of what kind of judge should replace the conservative Scalia on the closely divided court should be part of the presidential campaign this fall. Asked whether it will be a major issue in his bid for re-election, Portman replied: “I don’t know.”

But in a Senate race that polls have indicated will be tight, the vacancy debate adds a divisive issue that could sway the November outcome by propelling black voter turnout to support Obama or by firing up social conservatives already unhappy about the high court’s decision last year to strike down bans on same-sex marriage in Ohio and other states.

At stake this November is not only the presidency but majority control of the Senate. Democrats need a net gain of four seats, and Ohio is one of five states along with New Hampshire, Wisconsin, Illinois and Pennsylvania where the GOP faces a tough challenge. Obama won all those states twice.

Republicans recognize that a high-stakes fight over a Supreme Court choice energizes the GOP base. Democrats are accusing Senate Republicans of obstructionism and counting on independent and moderate voters to be turned off by any attempt to deny an Obama nominee a confirmation hearing, let alone a vote.

Outside an early voting center in north Columbus this week, Democratic activist Marc Gofstein said candidates’ positions on the Scalia vacancy are going to be an issue in the Senate race.

“Last I checked, President Obama doesn’t leave office until Jan. 20,” he said. “And my personal opinion on this is, if it was a Republican, this wouldn’t be an issue for Portman. He’d be pushing it like crazy.”

Speaking just outside the federal office building in downtown Cincinnati, Elizabeth Flanary, 48, said she can understand Obama’s interest in filling an important vacancy — and also Portman’s desire to delay the process.

If Obama proceeds with a nomination, “it should be someone who is neutral, since the court is divided and it could swing the other way,” she said.

An added twist is that Ohio Gov. John Kasich is seeking the Republican presidential nomination.

“I like our governor,” Flanary said. “I’d like to see him get elected president and get to appoint a Supreme Court justice.”

Portman, a former congressman and White House budget chief for George W. Bush, has been stockpiling campaign contributions at a state-record rate, closing last year with $10 million in donations and has $12.2 million cash on hand in this closely divided state.

John Green, director of the University of Akron’s Bliss Institute of Applied Politics, said independent voters tend to be “good government types” who will be turned off by too much political maneuvering on the court issue.

“I think that failing to have a confirmation process in Washington is very likely to be seen as just politics,” Green said.

James Campbell, a University of Buffalo political scientist, said there’s a strong chance the drama will conclude before November’s election and that Obama’s determination to proceed will do nothing but fuel the nation’s political divisions.

“The president has started the Kabuki dance,” he said. “They’re all going to go through the motions, it’ll all play out and we will be back at square one when the process is done — except there will be a lot of blood on the floor.”

Sewell reported from Cincinnati.