WASHINGTON - They came to honor Dr. Martin Luther King's dream, a tapestry of humanity with all shades of skin and from all walks of life. Yet there was something missing from the Lincoln Memorial this week: Republicans.

Fifty years ago, Democrats and Republicans stood shoulder to shoulder, demanding equal rights for African-Americans. But during the past week of commemorations of this formative American moment, the two parties barely interacted, each organizing its own events and delivering its own interpretations of King's dream.

Call it political segregation.

"No one even thought you should put a fig leaf over it," said Mary Frances Berry, a University of Pennsylvania history professor and former chairwoman of the U.S. Commission on Civil Rights.

"It's so obvious that it's partisan," Berry said. "I'm not sure that's good politics or the way to get anything done."

How did this happen? And how will it affect the effort to solve America's remaining racial challenges?

As the anniversary ceremonies unfolded, both sides said more unity is needed to fully realize King's dream - yet they showed few signs of wavering from positions that have been forged from decades of political warfare. And today's challenges - disproportionately high black poverty, unemployment, and education gaps - are more slippery than those of 1963, when the simple cry for equal rights drew support from both parties.

Robert J. Brown, who worked with King in the civil rights movement and then served as an aide to President Richard Nixon, said the 1963 March on Washington was "totally open. I was there in the middle of it. You had Democrats, Republicans, whites, blacks."

Today, "nobody wants to compromise anymore. They feel like their way is the only way," Brown says. "Well, I got news for you. In the history that thank God I have been somewhat a part of, that I came through, it was about compromise."

The two biggest commemoration events were a march last Saturday, and the ceremony Wednesday where President Barack Obama spoke from the same spot as King did precisely 50 years before.

Both were organized by a coalition of African-American advocacy groups closely tied to the Democratic Party, such as The Martin Luther King Jr. Center for Nonviolent Social Change, which is run by King's children; the NAACP; the National Urban League; and Rev. Al Sharpton's National Action Network.

The King Center invited both former President Bushes and all the Republican Congressional leaders to speak Wednesday. The elder Bush is too infirm to travel; his son declined due to a recent heart procedure but sent a statement of support. Other invited Republicans also declined, citing prior engagements.

Congress was in recess last week; it held a bipartisan ceremony in July. On Monday, the Republican National Committee held a commemorative luncheon that focused on attracting more minority voters to the GOP. Representatives from the NAACP and Urban League attended, but no Democrats were featured as speakers.

Raynard Jackson, a black Republican who helped organize the GOP event, said Democrats were using the occasion to foster high black and Hispanic turnout in the 2014 elections.

"They will instill and incite fear in the black and Hispanic community, use the march as a platform to talk about white folks, racism and Republicans," he predicted before Wednesday's event.

Few, if any, of the Wednesday speakers mentioned the GOP by name. But the atmosphere was thick with politics, and the word "fight" was uttered just as much as "dream."

Speaker after speaker urged battle against voter identification laws that disproportionately prevent black people from voting, the recent Supreme Court decision to neuter key provisions of the Voting Rights Act, and policies such as "stop and frisk" and "stand your ground."

Who supports these policies? Republicans.

From the podium Wednesday, Sharpton said that King fought and defeated Jim Crow segregation, and now "we come as the children of Dr. King to say we are going to face Jim Crow's children."

"Jim Crow had a son called James Crow Jr. Esq. He writes voting suppression laws and puts it in language that looks different but the results are the same," Sharpton said.

He concluded by saying: "We gon' keep on fighting until the dream is a reality."

In an interview, Sharpton said Republicans may have been reluctant to be associated with the advocacy leaders, union presidents and Democratic politicians featured in the ceremonies.

"They have become so intimidated by the Tea Party and far right wing, they fear to even be seen with some of us," he said.

On the other hand, event organizers might have been reluctant include conservative speakers who believe King's dream is already a reality and his fight has been won, said Charlton McIlwain, a New York University professor and author of "Race Appeal: How Candidates Invoke Race in U.S. Political Campaigns."

"Perspectives on race and dealing with race are so intimately intertwined with the political parties, much more than they were 50 years ago," McIlwain said. "It comes down to a fundamentally different view of what race means and how we solve problems that involve race."

It was not always so.

From the post-Civil War Reconstruction period until President Franklin Delano Roosevelt's New Deal, most black people who were allowed to vote went Republican - the party of Lincoln, the Great Emancipator. Racist Democrats ruled the South.

A shift began during the 1960 presidential election, when Democrat John F. Kennedy faced Richard Nixon. After Kennedy helped release Dr. Martin Luther King from a trumped-up jail term, he got 78 percent of the black vote - more than any previous Democrat, according to Berry, the history professor.

In 1964, the Republican candidate was Barry Goldwater, who opposed civil rights legislation. He lost to Democrat Lyndon Johnson, who then harnessed a coalition of northern Democrats and liberal Republicans to pass the historic laws meant to fulfill King's dream.