50 years after march, Obama aims to define his role in a new front in the fight for equality "I could see them coming across the stage," Jackson said. Those caskets we marched behind. And the people who made that night possible couldn't afford to be there. They would be either dead or injured or poor. Obama's relationship with the American civil rights movement has been a vexing one, challenging a cool, intellectual president to engage the memories and expectations of pioneers who marched, resisted and, in some cases, died before his birth. On Wednesday, the arc of that relationship will reach from Grant Park to the steps of the Lincoln Memorial. On the spot where the Rev. Martin Luther King Jr. described his dream 50 years ago, Obama will define a new front in the fight for equality and identify the mounting threats to progress emerging today. In preparing for the address, Obama has assembled civil rights advocates at the White House to discuss state efforts making it harder to register to vote and cast ballots nearly five decades after passage of the Voting Rights Act. He has met with faith leaders. He has phoned Rep. John Lewis , the Georgia Democrat who marched at Selma, Ala. According to those he has spoken to, Obama will say that gays and lesbians, women's rights advocates, immigration activists and African Americans must come together as a coordinated political movement to defend rights in peril, particularly at the ballot box, and to secure new ones in such areas as marriage equality and criminal sentencing. Although the groups are at odds on some issues, the broader demand for equal treatment unites many of them in ways that the political movements of the 1950s could not. But King spoke as a leader outside the government, affording him a certain rebel's freedom. Obama, as president, is searching to define his own role, constrained by his sense of race and by his responsibility to constituencies beyond King's. "I don't think people want him to be the black president or speak to black issues," said a senior administration official, who spoke on the condition of anonymity to describe Obama's thinking about the speech and the moment it presents. "He's the president of the United States; he's not a civil rights leader," the official continued. So, his message will be far broader, directed at the country as a whole. Obama has often declined to distinguish problems facing black communities from those confronting whites, Latinos and others, sometimes disappointing his mostly adoring African American supporters. He brushed aside the historic nature of his election early on, saying that "there was justifiable pride on the part of the country that we had taken a step to move us beyond some of the searing legacies of racial discrimination."