Conservative talk show host and one-time presidential hopeful Alan Keyes is expected to announce Sunday that he will run for the open Senate seat in Illinois, ending a prolonged and tortured candidate search for state Republicans.

Illinois GOP sources confirmed Thursday that Keyes has told members of the Republican State Central Committee he will accept their nomination offer and planning is already under way for a kickoff rally to be held Sunday.

Republican leaders offered the nomination Wednesday night to Keyes, a resident of Maryland who first surfaced as a possible Senate candidate earlier this week.

“They weren’t going to offer it to him until they got assurances from him that he’ll do it,” said one Illinois GOP operative.

Keyes, 53, will replace millionaire investment banker-turned-teacher Jack Ryan, who ended his bid in June amid embarrassing sex club allegations contained in previously sealed custody papers.

Ballots are due to be certified by state election officials at the end of the month, and under state law Keyes would have to live in Illinois only by Election Day.

Keyes’ entry into the race sets up a historical matchup in November against Democratic state Sen. Barack Obama. It will be the first time two black candidates representing major parties have faced off in a Senate contest.

Sen. Peter Fitzgerald (R-Ill.) is retiring after one term in office.

Obama’s national profile skyrocketed after he delivered the keynote address at the Democratic National Convention in Boston last week. He has raised more than $10 million in the race so far and had $3.4 million left in the bank at the end of June.

Obama welcomed Keyes into the race this week. Both men hold degrees from Harvard and are polished debaters.

“I hope over the next 90 days we can give the voters of Illinois a campaign they can all be proud of,” he said in a statement.

Keyes has run for Senate twice before in Maryland. In 1988, he challenged Sen. Paul Sarbanes (D) and lost 62 percent to 38 percent. Four years later he was defeated by Sen. Barbara Mikulski (D), 71 percent to 29 percent, after spending $1.2 million on the race.

In 2000, he unsuccessfully sought the GOP presidential nomination.