(Updates with polls closing)
By Mary Wisniewski
CHICAGO, March 18 (Reuters) - Illinois Republicans are voting on Tuesday to pick a candidate to challenge a Democratic incumbent governor they say is weak enough to give their party a shot at taking back an office long held by Democrats.
Four Republicans are running in Tuesday's primary election for a chance to unseat Governor Pat Quinn, who is viewed as honest but not forceful in a state whose prior governor is in prison for corruption.
"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."
Polls were closed at all but a few Illinois precincts as of 7 p.m. CDT. A few stayed open late due to delayed openings.
Turnout appeared low on Tuesday, despite sunny weather, compared with the last two elections, which saw turnouts of 23 percent to 24 percent, according to Illinois State Board of Elections assistant executive director Jim Tenuto.
"It just seems to be a lack of interest, a lack of excitement," Tenuto said.
An absence of big contests on the Democratic side seemed to be curbing enthusiasm, according to election officials.
Voters in the home state of President Barack Obama have chosen a Democratic governor in every election since 2002. But this year Republicans see the possibility of victory.
The Republican front-runner ahead of Tuesday's primary was wealthy venture capitalist Bruce Rauner, a political neophyte who has poured $6 million of his own money into the campaign. Those funds as well as millions in private donations have paid for a blitz of radio and TV ads that have helped push him past three more experienced opponents.
According to a recent Chicago Tribune poll, Rauner is 13 points ahead of his closest rival, 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.
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.
"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 and a former Chicago alderman.
Quinn is expected to win the Democratic primary handily - his only challenger on Tuesday is Tio Hardiman, a former leader of a Chicago anti-violence group who was dismissed after being arrested for domestic battery. That charge was later dropped.
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 the race for the Republican nomination for U.S. Senator from Illinois, state Senator Jim Oberweis, a multimillionaire dairyman, is running against businessman and political newcomer Doug Truax.
The winner of that contest will have the imposing task of taking on U.S. Senator Dick Durbin, the Senate's No. 2 Democrat, who has served in that body for 17 years.
Oberweis is better known and better funded, and was leading Truax by 52 to 15 percent in a February Chicago Tribune poll, though the Tribune endorsed Truax. Durbin is favored in the November general election.
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. He is likely to win, Simpson said. (Editing by Sharon Bernstein, Lisa Shumaker, Matthew Lewis and Cynthia Osterman)