More than three-quarters of U.S. Muslims support President Obama, a distinctly mixed blessing for a White House trying to combat rumors that Obama is a secret Muslim.

A new Gallup Poll shows that Obama got his highest approval ratings from U.S. Muslims, at 78 percent, for the first half of 2010. That’s down slightly from 86 percent over the same period last year, but still higher than any other American religious group.

Obama scored the lowest approval ratings from Mormons, at 24 percent, who tend to be Republican. Obama’s ratings were 43 percent among Protestants, 50 percent among Catholics, 64 percent among Jews and 63 percent among the nonreligious, atheists and agnostics.

Overall, the Gallup Poll revealed Obama’s overall national approval rating at 48 percent, down from 63 percent during the first half of 2009.

Gallup analysts said the overall trends — Muslims on top, Mormons at the bottom — remain consistent, but the recent controversy over the Islamic center near Ground Zero has elevated issues around U.S. Muslims.

A recent Pew poll found that one in five Americans mistakenly believe Obama is a Muslim, putting the White House on the defense about the president’s prayer life, religious observance and supposed ties to Islam.

Obama was born to a Muslim father who became an atheist, and a Christian mother who became an agnostic. Obama became a Christian as an adult at a predominantly black United Church of Christ congregation in Chicago.

The Gallup Poll was based on phone interviews with more than 276,000 U.S. adults, and has a margin of error of plus or minus 1 percentage point. Sampling error for smaller groups, including Muslims, varies.