More than half of Americans say same-sex marriage should be legal, garnering the same rights as traditional marriages, according to a newly released Gallup poll showing for the first time since Gallup began tracking the issue in 1996 support by a majority of Americans.

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The increase since last year — when 44 percent versus this year's 53 percent supported legalizing same-sex marriages — came exclusively among political independents and Democrats. Views of Republicans didn't change from last year, with 28 percent supporting gay marriage both years.

The results are based on Gallup's May 5-8 Values and Beliefs poll, which has tracked attitudes toward legalizing same-sex marriage every year since 2004, adding to its initial polling on the topic in 1996 and 1999. [ Same-Sex Marriage Debate Has Roots Going Back Centuries ]

This year's 9-point increase in support for same-sex marriage is the largest year-to-year shift yet measured over this time period. For instance, two-thirds of Americans were opposed to legalized same-sex marriage in 1996, with 27 percent in favor. By 2004, support had risen to 42 percent and, despite some fluctuations from year to year, stayed around that level of support through last year.

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The younger age groups showed the most support for legal gay marriage, with 70 percent the 18- to 34-year-olds indicating support, compared with 39 percent of those 55 and older.

Those who attend church less frequently showed more support than avid churchgoers for legal same-sex marriage. Greater support also was found among Catholics than Protestants, and among the unmarried. [ 5 Myths About Gay People Debunked ]

The cause of the uptick in support — at least among young Americans and Democrats and independents — is not known. But Gallup points out one factor may have been a reversal of the "don't ask, don't tell" policy; within the past year President Obama signed a law allowing gay and lesbian members of the military to openly reveal their sexual orientation, rather than keeping it hidden. Two-thirds of Americans indicated support for the policy change in 2009 and 2010, according to Gallup.
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However, most legislation on legalizing same-sex marriage occurs at the state level. Currently five states — Connecticut, Iowa, Massachusetts, New Hampshire and Vermont, plus the District of Columbia — allow legal same-sex marriages. Most of the remaining states outlaw it.

"At the moment, those advocating changes in constitutions and laws to allow same-sex marriage in additional states can take heart in the apparent shift in national sentiment in their direction," Gallup wrote in a statement.

The results are based on telephone interviews conducted May 5-8, 2011, with a random sample of 1,018 adults, ages 18 and older, living in all 50 U.S. states and the District of Columbia. The results were weighted for various factors to give nationally representative numbers.