(CNN) In August 1985, 42-year-old Delaware Sen. Joe Biden was preparing for his first presidential bid. Courting southern support, Biden traveled to Mississippi to honor the 84th birthday of Democratic Sen. John Stennis, who had served since 1947, at a celebration attended by thousands , including 10 Senate colleagues.

Now in his third run for the presidency, Biden presents his ability to respect and work with ideological opponents as an asset that could return the country to a bygone era of bipartisanship and consensus. His long record on civil rights gives him much to campaign on, as does his service as vice president to the nation's first black president.

Yet Biden's outreach to and past praise of senators who opposed civil rights and supported segregation in the 1950s and 1960s has come under increased scrutiny in a Democratic Party that has shifted dramatically in the years since. News reports have questioned how his eulogy of South Carolina GOP Sen. Strom Thurmond in 2003 would fare with a new generation of voters today. In April, Biden eulogized former South Carolina Democratic Sen. Ernest "Fritz" Hollings, who evolved from a segregationist to a supporter of civil rights who endorsed Rev. Jesse Jackson for president in 1988.

To better understand those past relationships and how Biden worked with his Senate colleagues from that era, CNN's KFile team reviewed a copy of Biden's 1985 speech from Stennis' senatorial papers at Mississippi State University . Stennis' files touch on that relationship, including handwritten notes, newspaper clippings, and letters that offer a deeper exploration of the warm relationship between the pair.

Stennis was an admired member of the Senate who served for a decade as chairman of the Armed Services Committee in the 1970s and was the first Democrat to condemn the actions of Republican Sen. Joseph McCarthy in the 1950s on the Senate floor, but he was also a leading member of a group of southern senators who frequently opposed civil rights bills and later the establishment of a federal holiday honoring Martin Luther King Jr. in 1983.

Stennis, Biden said in the 1985 speech, was a mentor whom he viewed as "a legend" upon entering the Senate in 1973, which was less than a decade removed from Stennis' votes against landmark civil rights legislation in the 1960s. Citing a speech Stennis gave at a caucus meeting during Watergate, Biden said that the Mississippi senator "cast a shadow that stretches far and wide."

Biden joined the Senate after the civil rights fights of the 1960s, but he established himself as a supporter of civil rights, including affirmative action and moving to end employment discrimination. In his 2020 campaign, Biden, along with other Democratic presidential hopefuls, has made President Donald Trump's divisive racial rhetoric central to his campaign, denouncing the President's response to a deadly 2017 white nationalist rally in Charlottesville, Virginia, when he said there were "fine people" on both sides. Biden has fared strongly in recent polls among black voters.

During his Senate career, Biden worked with prominent opponents of civil rights like Thurmond and Democratic Sen. Jim Eastland, Stennis' longtime Mississippi colleague.

CNN has previously reported on other letters from the 1970s that show Biden appealing to senators who supported segregation in the 1950s, seeking their support in his legislative fight against school busing. Like Biden, Stennis and Eastland were opponents of busing as a means to desegregate schools. A spokesperson for Biden recently told CNN that the former vice president's opposition was because he felt mandatory busing was an insufficient means to equal opportunity for students.

In a statement to CNN, Andrew Bates, a spokesperson for the Biden campaign, did not directly address Biden's relationships with Stennis, Eastland and Thurmond, but said, "Joe Biden's record shows that advancing civil rights and uprooting systemic racism have been defining causes of his entire life. In the United States Senate, he championed the Voting Rights Act -- securing multiple extensions of that critical law -- fought to outlaw housing discrimination, and took on the big banks over redlining. As Vice President, he fought voter suppression efforts and helped reduce the federal prison population by over 38,000."

Biden's close relationship with Stennis

Stennis, who was first elected senator in 1947, had a long record as an ardent opponent of civil rights. In the 1950s and 1960s, he opposed desegregation of public schools , the Civil Rights Acts of 1964 and 1968, and the Voting Rights Act of 1965. Like other prominent opponents of civil rights and school integration, Stennis claimed to oppose such laws on the basis of states' rights. He was noted for using less inflammatory rhetoric in opposing civil rights than some of his Senate colleagues.

Biden would go on to defend Stennis and Thurmond from charges of racism later in their lives, calling them captives of their eras who changed with the times. In his autobiography, "Promises to Keep," Biden wrote Stennis turned away from his past.

"In the years I'd known him, John Stennis turned away from his segregationist past," Biden wrote. "There are a whole lot of people who see his transformation as a journey of political expedience. I choose to see it as a journey of the heart -- a sincere desire to reflect the more generous instincts of Mississippians, and to honor the aspirations of all Americans."

When Biden first joined the Senate, he said Stennis befriended him shortly after he had lost his wife and one-year-old daughter in a car accident. Biden recalled in his book that Stennis asked him why he had run for office. "Civil rights, sir," Biden wrote he told Stennis. Biden recalled Stennis responded, "'Civil rights? Good,'" he said. "'Good. Good. Glad to have you here.'"

The pair, according to Biden in his Senate farewell address, would become close friends, at one point sharing a hospital room in the 1980s at Walter Reed Army Medical Center when Stennis had cancer and Biden had surgery to correct a brain aneurysm.

Biden later wrote vividly of an encounter with the older senator in 1989 when Stennis retired and Biden moved into his old office. Biden recounts the occasion in detail, including Stennis' very personal take on civil rights.

"'You see this table, Joe? This table was the flagship of the Confederacy from 1954 to 1968. We sat here, most of us from the Deep South, the old Confederacy, and we planned the demise of the civil rights movement,'" Biden wrote Stennis said. "'And we lost. And, Joe, now it's time that this table go from the possession of a man who was against civil rights to a man who was for civil rights.'"

"'The civil rights movement did more to free the white man than the black man,'" Stennis said, according to Biden.

"He could see me looking at him, confused, and he pounded on his chest. 'It freed my soul,' he said. 'It freed my soul.'"

Still, Stennis would officially make no apologies for his past opposition to civil rights bills upon his retirement. In the 1980s, he said he had believed that they "were too abrupt, went too far and were out-of-line." Stennis added, "We finally got out of that extremism. It takes time, takes time to make adjustments."

Historians say relationships made sense for the time

Historians who spoke with CNN's KFile said Biden's relationships with these senators made pragmatic sense for the time.

"These were powerful and influential people and they, you know, they certainly were not going anywhere," said Jason Ward, a professor of history at Emory University. "This also informs Biden's kind of wistful insistence on returning to some sort of era of bipartisanship, within his political lifetime -- he's not simply talking about Democrats and Republicans getting along. He's talking about getting along with, an extinct generation of Democrats."

In Biden's notes to Stennis , which KFile saw in the documents kept at Mississippi State University, he reflected on what occupying his friend's office meant personally.

"I have bittersweet feeling about moving into an office of such distinction. The bitterness relates to the knowledge that you will no longer be in the Senate and the sweetness to the knowledge that I will have a constant reminder of the man I viewed as a hero from the time," Biden wrote. "I sat next you at your conference table on my second day as a U.S. senator. You are a great man Senator Stennis and I will try to live up to the great and honorable tradition of the man who last occupied the office."

In his farewell address to the Senate , Biden reiterated that he got started in politics because of civil rights, and never expected he'd develop such strong friendships with senators like Eastland, Stennis, and Thurmond.

"I never thought I'd develop deep personal relationships with men whose position played an extremely large part in my desire to come to the Senate in the first place to change what they believed in -- Eastland, Stennis, Thurmond," Biden said. "All these men became my friends."

In one January 1975 letter, after Stennis helped Biden secure a seat on the Foreign Relations Committee, Biden wrote, "I will make every effort to fulfill your confidence" and that he hoped Stennis would "share with me your views on foreign policy matters" when they came up in committee. In April of that year , Biden told Stennis in a letter "I shall always treasure your writing" and that "observations coming from you can only be flattering and estimable."

And in 1980, Biden wrote a brief but admiring letter to Stennis where he said, "There a few 'gentlemen' that I have ever had the pleasure of getting to know and work with. You have increased the field by one. You are quite a man."

In his 1985 birthday speech for Stennis , Biden praised him as "an opponent without hate, a friend without treachery, a statesman without pretense, a victim without any murmuring, a public official without vice, a private citizen without wrong, a neighbor, as you all know, without hypocrisy, a man without guilt -- a senator whom future senators can study with profit for as long as there is an America."

Comparing Stennis to Stonewall Jackson in that speech, Biden said, "It was said of Stonewall Jackson, 'He is an avalanche from an unexpected quarter, a thunderbolt from a clear sky, and yet his character and will make him a stone wall and more of a stone wall than any man I've ever known.'"

Biden continued: "And, Mr. Chairman, when you stand on the floor of the Senate and you point that finger and you raise your voice, it's like a bolt from a clear sky, and when you speak, everyone listens. And as all of my colleagues have said here today, and you'll hear others say more of, he truly does stand like a stone wall, he is the rockbound integrity of the United States Congress."

Biden also recalled receiving a letter from Stennis soon after he delivered his first speech on the Senate floor.

"And then he wrote me a letter that I have hanging on my wall, and I've shown it to him, it's framed. He said, 'Today, in the United States Senate, I've witnessed a young man who has become a senator. You stood like a stone wall.'"