Coretta Scott King, widow of Martin Luther King Jr., walks a picket line to protest apartheid at the South African Embassy in Washington on Nov. 29, 1984. (Charles Tasnadi/AP)

When Sen. Elizabeth Warren (D-Mass.) used Coretta Scott King's words to express her opposition to President Trump's attorney general nominee, it was hardly the first time a Democratic politician saw the importance of King as a civil rights symbol.

At the height of the movement nearly 60 years ago, Martin Luther King Jr.'s wife was contacted by another senator from Massachusetts. It's a call some historians say played a role in solidifying what is now one of the most loyal bases of the Democratic Party — black voters.

It was October 1960. King was a pregnant wife whose husband had been arrested on a traffic charge while leading a protest in Atlanta. John F. Kennedy was a presidential candidate who was facing a tight race against Richard Nixon, and who understood that there were a significant number of black votes to be won, particularly in the industrial states of the North.

At that time, black voters tended to vote for the party of Abraham Lincoln, a Republican, and were skeptical of the wealthy senator from the Northeast.

Kennedy and a few people in his campaign saw an opportunity.

Harris Wofford, a Kennedy aide who later became a White House civil rights special assistant, saw the merits of reaching out to Martin Luther King Jr.'s wife and spoke to Kennedy's brother-in-law, Sargent Shriver, about it.

A public statement was too politically risky, so they opted for a discreet phone call, Harvard University history professor Fredrik Logevall said. Shriver, knowing the idea would be frowned upon by other aides, waited until he was alone with Kennedy and urged him to make the call.

Kennedy didn't need to be persuaded, Logevall said.

As King recalled in her memoir, “My Life, My Love, My Legacy”:

“May I speak to Mrs. King?” Sargent Sriver said. “Please hold for Senator Kennedy.”

After a brief greeting, Senator Kennedy expressed his concern for me and Martin. “I know this must be very hard for you. I understand you are expecting your third child, and I just wanted you to know that I was thinking about you and Dr. King. If there is anything I can do to help, please let me know.”

Of course I told him that I appreciated his concern and would welcome any assistance.

After the call, things happened fast. I began to hear encouraging news about Martin's release. Even A.D. [Martin Luther King Jr.'s brother] said, “I'll bet you he'll be out by tomorrow night.”

A.D. was right.

After Robert F. Kennedy, who was serving as his brother's campaign manager, called the governor of Georgia and the judge in charge of the case, Martin Luther King Jr. was freed.

Because of that phone call to Coretta Scott King, Kennedy gained the endorsement of Martin Luther King Sr., a lifelong Republican and a Baptist preacher who was supporting Nixon.

“Because this man was willing to wipe tears from my daughter[-in-law's] eyes, I've got a suitcase full of votes, and I'm going to take them to Mr. Kennedy and dump them on his lap,” the elder King said.

Voters, too, took notice.

The call, a shrewd move on Kennedy's part, turned out to be a consequential gesture that reverberated within the black community, Logevall said. What was initially meant to be discreet phone call became public, and in late October 1960, Kennedy made a statement about it at an impromptu news conference.

“She is a friend of mine and I was concerned about the situation,” Kennedy said.

More than 70 percent of black voters voted for him, according to the John F. Kennedy Presidential Library and Museum.

“I think he had genuine sympathy for the fact that King had been imprisoned without real cause, and it was very good for his campaign,” said Robert Dallek, a historian who focuses on American presidents. “They're not mutually exclusive.”

More broadly, Kennedy's personal intervention in the plight of a civil rights leader played a role in the shift of black voters from the party of Lincoln to the Democratic Party, Dallek said.

It started with Franklin D. Roosevelt's New Deal, which created economic programs for the unemployed following the Great Depression. African Americans, the disproportionately affected group, benefited from those programs. So although Roosevelt, a Democrat, had shied away from speaking out about civil rights and against lynching in the South, his New Deal paved the way for the significant shift of the black vote from the Republican Party, Dallek said.

Kennedy, Dallek said, deepened and strengthened that hold.

“I guess you can say it's one step on that trajectory,” Logevall said.

Almost four years after the phone call, Kennedy's successor, Lyndon B. Johnson, signed the Civil Rights Act into law.

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