Former President Barack Obama Barack Hussein ObamaObama calls on Senate not to fill Ginsburg's vacancy until after election Senate Republicans face tough decision on replacing Ginsburg Cruz: Trump should nominate a Supreme Court justice next week MORE marked Martin Luther King Jr. Day with a tweet on Monday honoring the slain civil rights leader for his “poetic brilliance” and “moral clarity.”

“Every so often, I re-read Dr. King’s Letter from a Birmingham Jail,” Obama wrote. “While some of the injustices may have changed, his poetic brilliance, moral clarity, and tests of conscience still reverberate today.”

“Take a moment to reflect on his righteous call,” he added in the message to his 112 million followers. His tweet also featured a link to a copy of the roughly 7,000-word letter King wrote from a jail in Birmingham, Ala., in 1963 that was published online by The Atlantic.

Every so often, I re-read Dr. King’s Letter from a Birmingham Jail. While some of the injustices may have changed, his poetic brilliance, moral clarity, and tests of conscience still reverberate today. Take a moment to reflect on his righteous call: https://t.co/oBdqFqdWA6 — Barack Obama (@BarackObama) January 20, 2020

The letter, which The Atlantic also published in its August 1963 print issue with the headline: “The Negro Is Your Brother,” was penned by King after he was arrested following the Montgomery bus boycott in 1956.

In the letter, King called for an end to “unjust” and “morally wrong” segregation laws, writing “all segregation statutes are unjust because segregation distorts the soul and damages the personality.”

On the 50th anniversary of the civil rights leader’s assassination in 2018, Obama joined Rep. John Lewis John LewisPelosi orders Capitol flags at half-staff to honor Ginsburg Kamala Harris: Black Americans have been 'disproportionately harmed' by Trump LWCF modernization: Restoring the promise MORE (D-Ga.), who protested next to King during the historic March on Washington, in discussing King’s legacy with a group of high schoolers in D.C.

At the time, Obama told Lewis: “Being on the right side of history isn’t always popular. And it isn’t always easy. You don’t know when things are going to break your way. You don’t know whether your labors will deliver.”

“Dr. King was controversial, but he studied and thought and crafted what he had to say. He knew, when he spoke, he was expressing a truth as well as he could know it,” Obama said.

“When you see something that is not right, not fair, not just, you have a moral obligation to do something, to say something. Dr. King inspired us to do just that,” Lewis added.