Indiana Governor Mitch Daniels, tipped as Republicans' best hope to oust President Barack Obama in next year's White House election, announced early Sunday he has decided not to run.

"Over the last year and a half, a large and diverse group of people have suggested to me an idea that I never otherwise would have considered, that I run for president," Daniels said in an email sent to supporters and published by The Indianapolis star newspaper.

"I've asked for time to think it over carefully, but these good people have been very patient and I owe them an answer. The answer is that I will not be a candidate."

Daniels cited family reasons for his decision not to toss his hat into the ring, saying that a "woman's caucus" of close relatives had nixed a White House run.

"On matters affecting us all, our family constitution gives a veto to the women's caucus, and there is no override provision. Simply put, I find myself caught between two duties. I love my country; I love my family more," he wrote.

Daniels' wife, Cheri, was said to be uncomfortable with a candidacy. She did not participate in her husband's 2004 gubernatorial campaign and has kept her public role to a minimum.

Speculation about her hesitancy over a White House run centered on the Daniels' marital history.

In 1993, Cheri filed for divorce after 15 years of marriage, moved to California and wed a doctor, leaving Daniels to care for four daughters, ages 8 to 14. Then she divorced again, and remarried Daniels in 1997.

Daniels made his announcement as another leading Republican, former Minnesota governor Tim Pawlenty planned to formally announce his Republican candidacy for president in Iowa on Monday.

It also came one day after radio talkshow host Herman Cain, who once ran the Godfather's Pizza chain, announced in an Atlanta rally that he was running.

The 65-year-old African American businessman has never held public office, but joined what has been a thinning field of Republican contenders after other leading prospects, former Arkansas governor Mike Huckabee and real estate tycoon Donald Trump, said last week they had decided not to run.

Despite not running, Daniels said he was still worried about the direction of the country under Obama's leadership, and hoped "to play some small part" in finding a political solution.

"I am deeply concerned, for the first time in my life, about the future of our republic," Daniels wrote.

"In the next few years, Americans will decide two basic sets of questions: Who's in charge here? Should the public sector protect and promote the private sector or dominate and direct it? Does the government work for the people or vice versa?

"I am confident that the answers will reaffirm the liberty and vitality of our nation," he added.

Indiana Democrats called his decision a "loss for Republicans."

"We've disagreed with Mitch Daniels myriad times, but there's no doubt that his decision not to enter this race is a loss for Republicans," the state Democrats wrote.

"Daniels would have brought a serious tone to a GOP field that's thus far been characterized by silliness and distraction."