In announcing his candidacy for the U.S. Senate on Aug. 8, Alan Keyes set up the nation's first all-black Senate contest. Naturally, that made history. Just as naturally, Mr. Keyes wasted no time making headlines, as well.

Within 24 hours, the conservative activist and former U.S. ambassador derided his opponent's support for ­abortion as "the slaveholder's position." Meanwhile, state Sen. Barack Obama, the Democratic nominee, needled Mr. Keyes for focusing on social issues instead of nuts-and-bolts Illinois concerns like jobs and health care. "It may be that [Mr. Keyes] just hasn't been here long enough," he concluded.

Mr. Obama's mild-mannered jab was only one of many "carpetbagging" remarks sure to dog Mr. Keyes, a Marylander scrambling to establish Illinois ­residency in order to run for the Senate. With Mr. Obama heavily favored, lushly bankrolled, and fawned over by the press, the Illinois GOP may be hoping merely to deflate the likely winner's swollen reputation.

"I don't think any Republican in Illinois expects Keyes to pull off an upset here," said Tripp Baird, Director of Senate ­Relations at the ­Heritage Foundation in Washington, D.C. "The best they can hope for is making Obama human again. He's had this coronation ride coming from the state Senate as a virtual unknown."

Indeed, Mr. Obama's rise from obscurity is breathtaking. Elected to the Illinois Senate in 1997, the Harvard grad vaulted through the chamber's Democratic ranks, emerging in just seven years as not only an all-but-certain U.S. senator but as the moderate new face of the national ­Democratic Party.

To Mr. Keyes, however, his opponent hardly qualifies as a moderate. He said it was the Democrat's well-disguised political beliefs that cemented his own decision to accept the state GOP's invitation to run in Illinois-especially a 2002 state Senate vote against protecting babies who were born alive after botched abortions. "We're not talking about arguments over fetal viability or health exceptions for the mother. This was a practice in which ­hospital workers would set aside a living newborn baby like garbage and let the child die. When I saw that depth of betrayal by a man claiming to be a ­moderate, I thought, can we just let this man waltz into the Senate unopposed, to represent the state of Lincoln?"

The answer, Mr. Keyes announced in his acceptance speech, was no. But in opposing Mr. Obama-if only to puncture the man's moderate reputation-might Mr. Keyes also be puncturing his own reputation as a straight-shooter?

In four runs for federal office, two each for president and U.S. Senate, Mr. Keyes has campaigned as a man of uncompromising principle. One of those principles: staunch support for federalism and the integrity of representative state government. Indeed, when Hillary Clinton conveniently decided to become a New Yorker in 2000, Mr. Keyes unloaded: "I deeply resent the destruction of federalism represented by Hillary Clinton's willingness to go into a state she doesn't even live in and pretend to represent people there, so I certainly wouldn't ­imitate it," he said at the time.

Now those words are coming back to haunt him. He insists this situation is ­different. Mr. Obama's covert liberalism, particularly his stand on abortion, is "threatening one of the founding principles of this country . . . that we respect the intrinsic worth of every human being no matter the circumstances of their birth. The one time when respect for state sovereignty has to be moderated is when national principles are at stake."

It's an argument Mr. Keyes has made before-sort of. During a Boise, Idaho, presidential campaign speech in May 2000, he discussed attempts in Vermont and San Francisco to legislate public acceptance of homosexuality. Under the constitutional federal system, state legislators have the right to pass such laws, he told supporters. But federalism ends, he suggested, when national mores are undermined: "If the people of San Francisco and Vermont try to abuse the [U.S.] Constitution in order to force other ­communities to accept the abandonment of moral principles they have embraced, that's where we need to draw the line."

Whether Mr. Obama's liberalism rises to the level of constitutional abuse-and thus to Mr. Keyes's threshold for trumping federalism-is a question ­Illinois voters may decide this fall. Some pundits already have decided that Mr. Keyes's current move from Maryland to Illinois is little more than a move from principled purism to political pragmatism. The day after he accepted the nomination, Los Angeles Times editors called his candidacy "wildly inappropriate," while The New York Times slammed Mr. Keyes for "rank hypocrisy."