President Obama is up again in Virginia in the see-saw presidential contest, according to a new poll that also shows him leading Republican challenger Mitt Romney in Wisconsin, but trailing in Colorado.

President Obama in the Roosevelt Room at the White House on Tuesday. (Susan Walsh/AP)

Obama had been ahead 50 percent to 42 percent in a poll Quinnipiac conducted in March. His lead was 47 percent to 42 percent in the school’s June survey. In July, voters were split 44 percent to 44 percent between the two candidates.

In the new poll, Obama won the support of 49 percent of voters, compared to 45 percent who supported Romney. The margin of error in the poll is 2.6 percent.

In Wisconsin, Obama leads Romney by a margin of 51 percent to 45 percent. But the survey shows Romney, the former governor of Massachusetts, ahead in Colorado, 50 percent to 45 percent.

“There is good reason why Virginia, Colorado and Wisconsin are considered swing states — and this data shows how close they are,” said Peter A. Brown, assistant director of the Quinnipiac University Polling Institute.

“Most voters say they have made up their minds. Nine out of 10 in each state say they are sure they will vote for the candidate they favor, which means that the pool of those who say they can be persuaded is pretty small.” At the same time, he added, “history tells us that many voters who say they are sure will change their mind in the next 90 days.”

In Virginia, the poll found that the U.S. Senate race between two former governors continues to be too close to call. Forty-eight percent of those surveyed favor Democrat Tim Kaine and 46 percent support Republican George Allen.

“Virginia’s U.S. Senate has been a dead heat since it began,” Brown said. “It’s pretty clear that whether George Allen or Tim Kaine becomes the Old Dominion’s next senator, it almost certainly will be by a razor-thin margin.”

Gov. Robert F. McDonnell (R), who is often mentioned as a potential running mate for Romney but endured a rocky General Assembly session this year, saw his approval ratings dip.

Fifty-two percent of Virginia voters approve of the job he is doing — the lowest approval rating of his tenure as measured by Quinnipiac. Twenty-nine percent disapprove. That represents a small decline, just outside the poll’s 2.6 percent margin of error, from his 55 percent approval rating in Quinnipiac’s July poll. It’s a more measurable slide from February, when McDonnell enjoyed a 58 percent score.

Even so, McDonnell’s job approval rating is higher than Obama’s in Virginia, where 47 percent approve of the president’s job performance.

Obama leads among women in all three states, by a margin of 54 percent to 40 percent in Virginia. Virginia’s independent voters favor Romney 50 percent to 43 percent.

Of interest to those parsing the numbers to see if McDonnell would be an asset to a Romney ticket in the crucial swing state: 60 percent of independents approve of McDonnell’s handling of the job, while 22 percent disapprove. Independents in Virginia give lower marks to Obama, with 39 percent approving of his job performance and 55 percent disapproving.

Women in Virginia approve of the governor’s handling of the job by a margin of 45 percent to 31 percent, while 50 percent approve of Obama’s performance and 44 percent disapprove.

Polling took place at a time when two of the three states were rocked by mass shootings. Voters were surveyed from July 31 to Aug. 6, beginning 11 days after 12 people died and 58 were injured in a mass shooting at a Colorado movie theater, and concluding one day after a gunman killed six people at a Sikh temple south of Milwaukee.

The poll found that voters in all three states do not believe tougher gun laws would prevent mass shootings such as the Colorado massacre. But by wide margins, they favor banning the sale of high-capacity magazines for guns.

Quinnipiac University, in cooperation with The New York Times and CBS News, surveyed likely voters in the three swing states via land lines and cellphones.

The poll surveyed 1,463 likely voters in Colorado, 1,412 in Virginia and 1,428 in Wisconsin. The margin of error in all three states is plus or minus 2.6 percent.