PRINCETON, NJ -- Barack Obama has a 48% to 39% advantage over Mitt Romney among independent voters in 12 key swing states. He first moved ahead of Romney among this group in February after being tied in January and trailing last year.

The results are based on the most recent USA Today/Gallup Swing States poll, conducted March 20-26, among voters in Colorado, Florida, Iowa, Michigan, Nevada, New Hampshire, New Mexico, North Carolina, Ohio, Pennsylvania, Virginia, and Wisconsin.

The poll preceded Romney's Tuesday victory in the key Wisconsin primary that made his nomination look increasingly inevitable. Romney's closest pursuer in the Republican nomination race, Rick Santorum, fared much worse versus Obama among independents in the Swing State poll, trailing 53% to 32%.

Overall, Obama leads Romney by 51% to 42% in the swing states, his first lead in five waves of interviewing in those states.

The movement toward Obama is essentially due to independents' changing preferences, because Democrats' and Republicans' preferences have been highly stable. Since October, Obama has averaged an 87% to 10% lead over Romney among swing-state Democrats, while Romney has averaged a 90% to 6% lead over Obama among swing-state Republicans.

Independent Women Lead Charge to Obama

Obama's standing against Romney has improved substantially in the swing states among women. Among independents, he has gained among both men and women, but more so among women. In combined data from October and December 2011, Obama trailed Romney by 11 percentage points among independent men and five points among independent women. In combined data from February and March, Obama has a one-point advantage among independent men and a 14-point advantage among independent women.

A closer look at the data in the previous table reveals that Obama's share of the vote has increased by eight percentage points among both independent men and women in swing states since last year. As would be expected, Romney's share of the vote has correspondingly decreased, but it has dropped significantly more among women (11 points) than among men (four points).

It is unclear how much of an impact the recent controversy over government policies toward contraception has had in the loss of support for Romney among independent women. Eight in 10 independent women in the swing states said they were not familiar with Romney's position on contraception, but those who were familiar disagreed with it by a 2-to-1 ratio. Independent women were more likely to have an opinion about Obama's views on contraception (58% were unfamiliar), and were divided about evenly between saying they agree or disagree with them.

Implications

Independent voters in the most competitive states may be the quintessential swing group, perhaps holding the key to victory for either Obama or his Republican opponent. Since last fall, their support has shifted toward Obama over his likely Republican opponent Romney, after previously favoring Romney. And it is those independent voters -- particularly women -- who are driving Obama's overall lead in swing states.

So while both campaigns will make considerable efforts to make sure their core supporters vote, the other big piece of their strategy would be finding the issues or themes that help win over independents in the states where either candidate has a reasonable chance of winning.

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