– President Obama, whose election fulfilled at least part of the promise of the Civil Rights Act signed into law 50 years ago by Lyndon B. Johnson, will mark the law's anniversary here this week along with three former presidents — Carter, Clinton and George W. Bush.

The LBJ Presidential Library Civil Rights Summit will explore the struggle so far and the work to be done, touching on not only race but issues including gay marriage, women's rights, immigration and — in a last-minute addition — disability rights.

"It's a milestone celebration on a very important topic — the effort to achieve equality — and that's why it matters. That's why those presidents are here. Lyndon Johnson, of course, is the central figure in making the laws comport with our values, and these presidents are here to express their respect for that," said Bruce Buchanan, a University of Texas ­political science professor.

Obama is scheduled to speak Thursday, and Bush later that day. Carter speaks Tuesday, and Clinton Wednesday. LBJ Presidential Library Director Mark K. Updegrove said nothing in Johnson's presidency and its sweeping ­domestic agenda affected the nation more profoundly than the Civil Rights Act, which prohibited discrimination on the basis of race, color, religion, sex or national origin.

"His chief legacy, the headline of his presidency, should be about civil rights," Updegrove said.

Obama, whose path to becoming the nation's first black president was made possible by civil rights strides, gave an important speech about race in the 2008 presidential campaign after controversy over his pastor Jeremiah Wright's incendiary comments and also spoke personally about race in the context of Trayvon Martin's shooting death.

Updegrove said that he hopes Obama "might use this as a platform to sound the central message that he wants to deliver as president on the subject of race."

Gov. Rick Perry, who's frequently at odds with Obama administration policies, was invited to the summit but is in the midst of a trip to the Republic of Palau as part of an expedition searching for missing-in-action World War II servicemen. Perry's communications director, Felix Browne, said Perry was committed to the Palau trip when he got an invitation to the summit on March 13.

"The governor is proud that Texas will host this 50th anniversary celebration of the Civil Rights Act, one of the great milestones in the history of our nation," Browne said.