When Sen. Barack Obama (D-Ill.) takes the stage tonight at Invesco Field at Mile High, it will seal a stunning achievement that many of his colleagues in the Congressional Black Caucus thought impossible at this time last year.

That Obama will become the first African-American presidential candidate of a major party on the 45th anniversary of Martin Luther King Jr.’s “I Have a Dream” speech only adds emotional punch to the moment.

But some black lawmakers are also nervous that demonstrating their pride too loudly could imperil Obama’s candidacy. On the brink of a seminal moment in the civil rights movement, they are taking pains to downplay its significance and keep the focus on Obama’s promise as a global leader.

“It’s not a black thing, it’s an American thing,” CBC Chairwoman Carolyn Cheeks Kilpatrick (D-Mich.) said. “I don’t want to make this a black thing. … It doesn’t fit there, and we’ll lose if we do that.”

Exactly how Obama will fit the anniversary of King’s speech into his own tonight is not clear, though he is expected to note the nation’s progress since that time. CBCers said they don’t think Obama will, or should, dwell on the topic of race — an issue that has proved both a boon and a burden for him. black voters mobilized in record numbers to help him secure the Democratic nomination. But the scandal this spring over controversial remarks by the Rev. Jeremiah Wright, Obama’s former pastor, threatened to derail his candidacy and prompted him to give a speech on how he views race relations in America.

The issue reared up again a month ago, when Sen. John McCain’s (R-Ariz.) campaign accused Obama of playing the race card after he said Republicans would try to sow doubts about him by reminding voters he “doesn’t look like all the presidents on the dollar bills and the $5 bills.”

CBCers said to win the White House, Obama must transcend the racial divide — a task they indicated they are loath to complicate by highlighting Obama’s ethnicity. Dwelling on his race, they said, diminishes his strength as a leader.

“I don’t think we’re nominating a CBCer. I think we’re nominating a capable, competent, qualified person who just happens to be from Chicago, who just happens to be a member of the United States Senate and just happens to be a member of the Congressional Black Caucus,” Rep. Al Green (D-Texas) said. “But I didn’t come here to nominate a CBCer. I came to nominate a capable, competent and qualified candidate who just happens to have some other qualities that I have something in common with.”

Kilpatrick said CBC members “obviously” are “most proud” of Obama’s achievement. “It’s obviously historic for us,” she said. But she said Democrats “lose his total value” if they focus on it.

One aide to a CBC lawmaker said the African-Americans in Congress are steering wide and clear of an “‘it’s our turn now’ message, which is divisive and polarizing.”

“They’re obviously excited — you can see it in their faces,” the aide said. “But they are also nervous. No one in this party thinks this thing is in the bag.”

Not every member of the caucus is shying from a celebration. House Majority Whip James Clyburn (D-S.C.), the nation’s highest-ranking black official and a veteran of the civil rights struggle of the segregated South in the 1960s, has publicly discussed how emotional he was when Obama sealed the nomination in June. Addressing the House Democratic Caucus on Wednesday, he said Obama’s nomination is an outgrowth of the the Voting Rights Act of 1964.

“I talk about what it means to me as a 68-year-old African-American who spent a lot of time in pursuit of the American dream,” Clyburn said in an interview. “To come to this point, and see this party, my party, which has been accused by the media of always taking African-Americans for granted, about to give an African-American the highest, the biggest, greatest prize this party has, to be in a position to be the No. 1 leader in the free world, that is important to me.”

As for the threat that Obama’s race could ultimately keep him from winning the presidency, Clyburn said he is not worrying.

“Race is obviously a problem for us in this country. We know that,” he said. “I don’t fret over those things. You recognize they’re real, but you keep going.”