COLUMBUS — The presidential race in Ohio is too close to call with 45 percent of voters supporting President Barack Obama and 44 percent backing Republican Mitt Romney, according to a poll released Thursday by Quinnipiac University.

Even if Romney were to pick U.S. Sen. Rob Portman of Cincinnati as his running mate, the numbers hardly budge. A Romney-Portman ticket garners 45 percent support while the Obama-Biden ticket gets 45 percent, the poll found.

“Adding Sen. Rob Portman to the Romney ticket produces no measurable change, despite speculation about Portman’s potential for helping Romney carry the key state of Ohio,” said Peter A. Brown, assistant director of the Quinnipiac University Polling institute.

But a few percentage points can make a big difference. In 2004 President Bush defeated Democrat John Kerry by 2.1 percent.

No Republican has ever won the White House without carrying Ohio.

Meanwhile, the U.S. Senate race is tightening. Republican challenger Josh Mandel edged up to 40 percent support, up from 36 percent in March. He still trails Democratic U.S. Sen. Sherrod Brown, who holds 46 percent support. Brown leads among independents 42-38 percent, the poll found. Both Mandel and Brown released new TV ads this week.

“Is Sen. Sherrod Brown hearing the footsteps of state Treasurer Josh Mandel?” Peter Brown said. “Ohio voters say 44-38 percent that Brown deserves another six years in the Senate, matching his six-point margin over Mandel, but short of the 50 percent any incumbent would like to have. Meanwhile, voters are getting to know Mandel. In February, 71 percent didn’t know enough about him to form an opinion. That ‘don’t know’ number is now 51 percent.”

Quinnipiac University surveyed 1,069 registered voters on land lines and cell phones between May 2 and May 7. The survey has a margin of error of plus or minus 3 percentage points.