By Mary Wisniewski

CHICAGO (Reuters) - A state appeals court on Monday threw the Chicago mayor's race into turmoil by ruling that front-runner and former White House chief of staff Rahm Emanuel did not qualify for the February ballot.

Emanuel immediately responded that he would appeal the ruling to the state Supreme Court and urged quick consideration. The ruling on Monday overturned decisions by a lower court and a Chicago elections board that allowed him on the February 22 ballot.

"I have no doubt that, in the end, we will prevail," Emanuel said at a news conference held at a downtown restaurant. "As my father always used to say, 'nothing is ever easy in this life.'"

"I do believe the people of Chicago deserve the right to choose who they want as the next mayor," Emanuel said.

Illinois law requires candidates for municipal offices be residents of the city for a year prior to elections. Emanuel, who resigned as President Barack Obama's White House chief of staff in October to run for mayor, was challenged on the issue by several Chicago residents.

The appellate court, in its 2-1 ruling, said that though Emanuel owned a house in the city, he rented it out and did not live there. The 25-page ruling cited legal and dictionary definitions of the word to "reside," which the court said means "to dwell permanently or continuously" -- and which it said Emanuel clearly did not do over the past year.

The court said the exception for residency while serving elsewhere on U.S. business is extended to voters, but not to candidates.

Emanuel said he ought to qualify for the ballot because he owns a home in Chicago and said his job as the president's top aide "counts as service to your country."

He asked the state's Supreme Court to consider the case quickly as his name had already been removed from absentee ballots, which could confuse voters.

"Of course it changes the entire complexion of the race," said Dick Simpson, a political analyst at the University of Illinois-Chicago.

Simpson said former Senator Carol Moseley Braun now is the nominal front-runner, although the ruling scrambles the race.

Emanuel's legal appeal could consume his campaign for weeks and remind voters that he had spent a lot of time away from the city, Simpson said. The current mayor, long-time Chicagoan Richard Daley, is retiring after 22 years in office.

A Chicago Tribune poll published last week showed Emanuel leading the race by a comfortable margin, with 44 percent compared to his nearest rival Braun at 21 percent.

If no candidate wins more than 50 percent of the vote in February, the top two vote-getters face each other in a run-off in April.

Braun has criticized Emanuel as an "outsider" and planned a news conference of her own this afternoon.

Emanuel raised $11.8 million for the campaign, from big names such as film director Steven Spielberg. Braun, by contrast, raised less than $500,000.

(Writing by Andrew Stern, Editing by Greg McCune)