"A poor choice of words conveyed to some the impression that I embraced the discarded policies of the past," Lott said in a statement issued Monday night. "Nothing could be further from the truth, and I apologize to anyone who was offended by my statement."
Lott's statement came "out of personal concern for the misunderstanding," his spokesman, Ron Bonjean, said.
Members of the House Congressional Black Caucus, at a news conference Tuesday, said Lott's apology was insufficient. "I'm very concerned and very upset that anybody who would issue such a statement would be in the leadership of this nation or the Senate," said Rep. Elijah Cummings, D-Md., who on Tuesday was elected the next chairman of the 39-member caucus.
Rep. Maxine Waters, D-Calif., warned fellow Democrats that it would not be acceptable to explain away such statements "and then at election time talk about why black Americans should turn out in large numbers."
Earlier Monday, former Vice President Al Gore, who served in the Senate with Lott and Thurmond, said Lott should withdraw his "racist statement" or face Senate censure of those comments.
Civil rights leader Jesse Jackson described Lott, a Mississippi Republican, as "an unrepentant Confederate who cannot speak for all Americans."
Gore, speaking on CNN's "Inside Politics," said the Senate should censure Lott. "It is not a small thing for one of the half-dozen most prominent political leaders in America to say that our problems are caused by integration and that we should have had a segregationist candidate," he said. "That is divisive, and it is divisive along racial lines."
Lott, a 14-year Senate veteran, returns to the position of majority leader next month because Republicans recaptured control of the Senate in November's elections. The leader of the majority party is the most powerful senator because of the leader's control of the Senate agenda.
At a party celebrating retiring Sen. Thurmond's 100th birthday, attended by hundreds of Thurmond's family members and friends from South Carolina, Senate colleagues and members of the Supreme Court, Lott said that when Thurmond ran for president on a states' rights, anti-integration ticket in 1948, Mississippi voted for him.
"We're proud of it," Lott said to applause. "And if the rest of the country had followed our lead, we wouldn't have had all these problems over all these years, either."
In a statement Monday before he apologized, Lott insisted his comments last week had been lighthearted and in no way endorsed Thurmond's positions of more than a half-century ago.
"This was a lighthearted celebration of the 100th birthday of legendary Sen. Strom Thurmond," Lott said in his first statement. "My comments were not an endorsement of his positions of over 50 years ago, but of the man and his life."
A call left at Thurmond's Washington office after work hours was not immediately returned.
Jackson, in a statement, said Lott should step down. "The civil rights movement was one of America's finest hours. Strom Thurmond's massive resistance to that movement, and his support in states like Mississippi, was one of one of history's low points. Trent Lott must not be allowed to tarnish that truth."
Kevin Martin, government and political affairs director of the African American Republican Leadership Council, said people were overreacting. "By no means was he endorsing segregation or anything like that," Martin said. "It was lighthearted. It was humorous." Martin said Lott wins 25 percent of the black vote in Mississippi, which he said couldn't happen if Lott were a racist.
Senate Democratic leader Tom Daschle also came to Lott's defense Monday, saying he had talked with Lott on the phone and accepted Lott's explanation that he hadn't meant for the remarks to be interpreted as they were.
"There are a lot of times when he and I go to the microphone and would like to say things we meant to say differently, and I'm sure this was one of those cases for him, as well," Daschle said.
Thurmond, then governor of South Carolina, received 39 electoral votes in his 1948 president bid, all from Southern states. He ran as a "Dixiecrat," breaking away from the Democratic party, and during the race remarked, "All the laws of Washington and all the bayonets of the Army cannot force the Negro into our homes, our schools, our churches." His campaign posters proclaimed "Now is the time to fight."
He entered the Senate in 1954 and became one of the South's most vocal opponents of integration. He opposed the Supreme Court's 1954 Brown vs. Board of Education school desegregation decision and filibustered against civil rights legislation.
Later in his career he changed his positions, hiring black staff members and helping promote blacks to federal judgeships.
Thurmond retired this year as history's oldest member of Congress and the longest-serving senator ever.
Lott has angered black groups in the past with his one-time support for the Council of Conservative Citizens, which Lott once appeared to praise as standing for "the right principles" but has since disavowed.
The group grew out of smaller segregationist organizations and supports an extremely conservative position on racial matters, including arguing that Martin Luther King does not deserve a national holiday.