By Mary Wisniewski

CHICAGO (Reuters) - Illinois Governor Pat Quinn coasted to a win in the state's Democratic primary on Tuesday, while the Republican primary to choose a candidate to challenge Quinn in November was too close to call several hours after polls closed.

Republicans are looking to Illinois as perhaps the party's best shot to take out a Democratic incumbent governor. Bruce Rauner, a wealthy businessman who has already pumped $6 million of his own money into the campaign, leads the four-person field of Republican candidates, according to preliminary results by Reuters U.S. Election Service.

While Quinn is seen as facing a difficult re-election fight five years after taking over from Rod Blagojevich, a fellow Democrat who is now in federal prison on corruption charges, he easily beat anti-violence activist Tio Hardiman with 73 percent of the vote, according to preliminary results released by the Chicago Tribune.

"Illinois will be one of the primary focuses of traditional Republican groups and groups that are interested in conservative economic policy," said Kent Redfield, emeritus professor of political science at the University of Illinois at Springfield. "This is an opportunity to flip a state from Democrat to Republican."

Meanwhile, in the Republican primary to take on veteran U.S. Senator Dick Durbin in November - the No. 2 Democrat in the Senate - Illinois State Senator Jim Oberweis, the millionaire owner of a dairy business defeated businessman and political newcomer Doug Truax for the Republican nomination.

"The state is in deep trouble. Our country is in deep trouble," Oberweis said in a victory speech.

Election officials have reported low turnout on Tuesday, which was 23 percent to 24 percent in Illinois in 2010 and 2012.

Voters in the home state of President Barack Obama have chosen a Democratic governor in every election since 2002.

Whoever wins Tuesday is expected to face a tough and expensive contest against Quinn in November, who despite low popularity ratings and Illinois' continuing fiscal problems will have strong union support.

Even before all the returns were in, Quinn was already running a TV ad against Rauner blasting him for saying that he was against raising the minimum wage. Rauner later modified his position.

"Quinn wasn't expected to win last time, but the groundswell of support from labor unions and regular folks who like him sort of surprised people," said Dick Simpson, a political science professor at University of Illinois-Chicago.

Rauner has used his considerable campaign war chest to buy a blitz of radio and TV ads that have helped him build his name recognition and push him past three more experienced opponents.

More than two hours after the polls closed in Illinois, two candidates, Republican State Senator Bill Brady and State Treasurer Dan Rutherford, conceded, and Rauner was in a two-man race with State Senator Kirk Dillard, who had served as chief of staff with popular former Republican Governor Jim Edgar.

Rauner has steered clear of social issues and focused on Illinois' troubled economy. He has also criticized other lawmakers, including Dillard, for taking union money.

A few Chicago-area voters who usually vote Democratic said they were taking a Republican ballot to vote against Rauner. Though Rauner is polling well among Republicans, he has stirred controversy because of his strong anti-union rhetoric and other issues, like pulling strings to get his daughter into a competitive Chicago high school.

Victoria Beal, 46, a Chicagoan who usually votes Democratic, went for Dillard on Tuesday. "It's not so much a protest vote as a pro-union vote," she said.

In one controversial Chicago contest, Democrat Isaac "Ike" Carothers, a former Chicago alderman who served prison time for bribery and tax fraud, is running for commissioner of the Cook County Board, which handles the court system and healthcare for the Chicago metropolitan area.

(Editing by Edith Honan and Lisa Shumaker)