Alan Keyes (search), the conservative Maryland political commentator whom Illinois Republicans turned to after weeks of searching for a replacement Senate candidate, agreed Sunday to run against a rising Democratic star who has only grown stronger amid GOP scandal and disarray.

"We do face an uphill battle, there's no doubt," Keyes told cheering supporters at a rally.

"So I'm not going to stand here and with tremendous ease promise you a victory. But I'll tell you what I will promise. I will promise you a fight!"

The Republican two-time presidential hopeful will face Democratic state Sen. Barack Obama (search) in the campaign to replace retiring GOP Sen. Peter Fitzgerald. Keyes' entry sets up the first U.S. Senate election with two black candidates representing the major parties, and seemingly assures Illinois of producing only the fifth black senator in history.

Dropping into the race from another state is an uncomfortable position for Keyes, who criticized Hillary Rodham Clinton (search) for moving to New York to make her 2000 Senate run. When asked last week how he felt about running for Senate in a state he had never lived in, he responded: "As a matter of principle, I don't think it's a good idea."

On Sunday, Keyes spent much of his speech discussing his love of Maryland and his deliberations over running in Illinois.

Keyes said he felt he should leave Maryland to "defend the land of my spirit and my conscience and my heart."

"If indeed that land is the state of Illinois, then I have lived in the Land of Lincoln all my life." he said.

Keyes, who turned 54 on Saturday, replaces Jack Ryan (search), who withdrew from the race June 25 amid embarrassing sex club allegations in his divorce records. Keyes, who has no Illinois ties, emerged as a candidate only recently, after a host of high-profile Illinois Republicans, from former Govs. Jim Edgar and James Thompson to former Chicago Bears coach Mike Ditka (search), declined to run.

Keyes' abortive presidential runs in 1996 and 2000 have helped make him widely known for his conservative views, which many Republicans say will match up well against what they say are Obama's liberal views.

Keyes opposes abortion and gay rights (search), wants to replace the income tax with a national sales tax and calls affirmative action (search) a "government patronage program."

He said he decided to enter the race after reviewing Obama's record on abortion, gun control and taxes.

"He has never seen a spending bill he couldn't find some excuse for and has never seen a tax increase he didn't like," Keyes said. "We find somebody who in the tradition of a lot of the liberals would rather that our children are educated in schools controlled by impersonal bureaucracy than in schools under the influence and control of the parents who love them and care about their future."

In a statement, Obama said as Keyes travels the state, he'll find that "families here are concerned about quality jobs, making health care more affordable and ensuring our children get the best education possible.

"And Illinoisans want a Senate candidate who will attack the problems they and their families face rather than spending time attacking each other," Obama said.

Keyes begins with a heavy disadvantage against Obama, a legislator from Chicago who has raised more than $10 million and delivered the keynote address last month at the Democratic National Convention.

Even before Ryan dropped out, Obama was considered a heavy favorite in Illinois, considered a Democratic-leaning state.

Obama has called the GOP choice of a black candidate "a hopeful sign for the country," but said voters might balk at an out-of-state candidate.

Keyes has until Election Day to establish residency in Illinois according to federal law. The party has until Aug. 26 to submit his name for the ballot.

Keyes told Republican leaders he would turn to his national base of supporters, as well as to national party leaders, for financial help.

Keyes still owes $524,169 from his two presidential bids, according to federal elections records. He also owes $7,481 in unpaid state income taxes in his home state of Maryland, according to court records.

The state filed a lien against Keyes in December 2001 for those unpaid taxes. Bill Pascoe, a Keyes adviser, said the tax bill was erroneous and Keyes only owed $152, which he paid on Friday.

Before deciding to run for elected office, Keyes -- who has a Ph.D. from Harvard in government affairs -- served in the U.S. Foreign Service, was appointed ambassador to the United Nations Economic and Social Council (search) by President Reagan (search) and served as an assistant secretary of state.

He first ran for the U.S. Senate from Maryland in 1988, winning 38.2 percent of the vote.

In 1992, he got 29 percent against Sen. Barbara Mikulski (search), D-Md., amid grumbling by some voters that he had been paying himself a salary with campaign funds. He told Illinois GOP State Central Committee members he won't do that this time, said committee co-chairman Stephen McGlynn.