Susan Page, USA TODAY

In battleground states, 64.2% of eligible citizens went to polls

Only 56.8% voted in the rest of the nation

In 1996, there was little difference in turnout between swing states and other states

Here's one more way swing states stand out: Their citizens are more likely to vote.

That may not be surprising given, in this year's presidential campaign, the battleground states were deluged by TV ads and targeted for sophisticated get-out-the-vote operations. After all that, 10 key swing states had significantly higher turnout than the rest of the USA, an analysis of data by the non-partisan Center for the Study of the American Electorate shows.

That turnout gap is growing, with potential repercussions for candidates down the ballot and the sense of connection some Americans have with their government.

"Increasingly, people in non-swing states don't think their vote makes any difference," says Curtis Gans, director of the center. That seems to be costing non-battleground states some of the boost in voter participation that presidential contests traditionally bring.

It is a distinctly American phenomenon. Because of the Electoral College system, presidential candidates now essentially ignore states that are solidly Republican or Democratic. They focus instead on the dwindling number of states that might swing one way or the other.

As recently as 1996, there was essentially no difference in turnout between the swing states and the other states plus the District of Columbia: 51.5% of eligible citizens in the battlegrounds voted; elsewhere, 51.4% did. In that campaign, the presidential candidates contested more states and their TV ads aired more broadly.

Differences in turnout have been steadily widening since then: 1.2 percentage points in 2000, 4.4 points in 2004 and 5.2 points in 2008. This year, 64.2% of eligible citizens went to the polls in the battlegrounds compared with 56.8% in the rest of the nation — a disparity of 7.4 points.

The battlegrounds used in this analysis are Colorado, Florida, Iowa, North Carolina, New Hampshire, Nevada, Ohio, Pennsylvania, Virginia and Wisconsin. All were targeted by President Obama and Republican Mitt Romney in 2012, although the list of swing states would have been somewhat different in each of the previous campaigns. (That makes the trend line more illustrative than precise.)

The conclusion by citizens in non-battleground states that their vote doesn't really matter could "add to the sense that some voters feel left out of politics," Democratic pollster Margie Omero says. calculates the turnout gap is likely to hurt Democratic candidates more than Republican ones because the people most easily discouraged from voting include such Democratic-leaning groups as racial minorities and young people.

"The good news is some of the new efforts at increasing turnout seem to be working: The ads, the person-to-person contact, the early voting," Omero says. Although residents in swing states in 2012 often decried the onslaught of TV ads, most of them negative, the commercials apparently did more to boost turnout than to turn off voters.

Gans says the findings strengthen the case for modifying the nation's unique political system so candidates have a reason to contest more states — perhaps by distributing Electoral College votes proportionately rather than winner-take-all in a state, or by allocating the votes by congressional district rather than statewide. Maine and Nebraska already do that.

Nationwide, 58.75% of eligible citzens voted in 2012, down from 62.29% in 2008. The turnout rate rose significantly in the District of Columbia, which permitted same-day registration. There was a rise in Massachusetts, presumably because of its hotly contested Senate race between Republican Sen. Scott Brown and the victorious Democratic challenger, Elizabeth Warren.

Turnout edged up a tick in Iowa, Colorado and Louisiana and fell everywhere else — by 1.9 points in battleground states and 4.1 points in non-battleground states. It dropped 8.52 points in New York and nearly 5 points in New Jersey, states that had been hit hard by Superstorm Sandy two weeks before Election Day.