Story highlights Peniel Joseph: It's no accident that Trump went after Lewis on the eve of MLK Jr. Day

Every president (until now) has taken racial justice as good for the republic, he writes

Peniel Joseph is the Barbara Jordan Chair in Political Values and Ethics and the founding director of the Center for the Study of Race and Democracy at the LBJ School of Public Affairs at the University of Texas at Austin, where he is also a professor of history. He is the author of several books, most recently "Stokely: A Life." The views expressed here are his.

(CNN) This year's Martin Luther King Jr. holiday, the last that will be commemorated during the Obama presidency, offers the opportunity for sober clarity about the present leavened by defiant hope for the future. The King holiday, a hard-won victory by veterans of the civil rights era alongside of black elected officials, became a crucial part of a new national consensus on racial equality.

Dr. King's complex iconography can be understood with a simple lesson: He brought racial justice into the mainstream as a fundamental element of American democracy.

On this score, King served as the civil rights movement's most important political mobilizer, a global Nobel Peace Prize-winning figure capable of bridging racial and economic divides by participating in bus boycotts in Montgomery, Alabama, writing a passionate letter from a Birmingham jail cell, leading a March on Washington, and traversing the Edmund Pettus Bridge in a demonstration that galvanized world opinion in support of the struggle for black dignity.

Peniel Joseph

In the shadow of continued moments of racial strife, the Martin Luther King, Jr. holiday represents America's political and rhetorical acceptance of civil rights as a moral and political good. President-elect Donald Trump's recent assaults on civil rights icon and Georgia Congressman John L. Lewis, however, signal the potential end of this consensus, diminishing our nation's moral stature on race matters at the precise moment we need it most.

Lewis is a genuine American hero, a humble and courageous foot soldier and civil rights leader, who suffered severe beatings in 1961 as a Freedom Rider and four years later during the "Bloody Sunday" march in Selma, Alabama. He is the last major speaker from the 1963 March on Washington living in America.