(Updates with background, details)

By Fiona Ortiz

CHICAGO, Nov 5 (Reuters) - Illinois Democratic Governor Pat Quinn on Wednesday conceded the election to Republican challenger Bruce Rauner after a $100 million mud-slinging race that has put a wealthy political newbie in charge of the financially troubled state.

Unofficial results show Rauner took 51 percent of the vote with close to 100 percent of precincts reporting. Quinn trailed by about 157,000 votes, taking some 47 percent. Libertarian Chad Grimm took 3 percent.

Earlier on Wednesday Quinn had said he would not concede until every single absentee and provisional ballot was counted. Polling in Illinois was plagued with delays on Tuesday and a mysterious campaign of automated phone calls targeting election workers, which is now under criminal investigation.

"It is clear that we do not have enough votes to win the election and therefore we respect the result, we respect what the voters did yesterday and I look forward to working with the new administration," Quinn told reporters at a very brief news conference in Chicago.

An hour earlier Rauner announced his transition team. With Democrats still firmly in control of the state legislature, he has pledged to work with House Speaker Michael Madigan to fix the state's fiscal woes.

Illinois' public employee pension system is underfunded by $100 billion and the state has the lowest credit rating of any U.S. state.

In the campaign 65-year-old Quinn had attacked Rauner, a 57-year-old venture capitalist with nine homes, saying he was a heartless businessman who does not care about the issues affecting average people.

Rauner portrayed Quinn as a tax-and-spend liberal responsible for the state's economic doldrums and for continued corruption problems in big state agencies. (Reporting by Fiona Ortiz and David Bailey; Editing by Eric Beech and Eric Walsh)