WASHINGTON — Sen. Joe Biden of Delaware didn't need to hear any more from the legion of critics lined up against his bill limiting federal courts' power to desegregate schools by busing students.

It was July 1977, there wasn't much time before such an order was to take effect in Wilmington — the largest city in Biden's home state — and he was eager to move the bill he had written with Delaware's senior senator, Republican William Roth, through the Judiciary Committee and to the Senate floor.

One of his aides, Gerry Doherty, had just told Ken Dixon, a staff member for liberal Sen. Birch Bayh, D-Ind., that the panel wouldn't hold more hearings to gather testimony from civil rights and labor groups opposed to the legislation when Biden walked up to the pair, according to a memo Dixon wrote at the time.

"You can put as many people in the hearing record as you want," Biden said, according to the memo, which has been preserved in the Birch Bayh Senatorial Papers section of the Modern Political Papers collection at Indiana University. "But you know it and I know it that if I don't get a bill out by September, it won't do me any good."

To Bayh’s staff, the incident demonstrated the intensity of Biden’s opposition to busing — earlier in his career, he had been more reluctant to bind courts — and how politically important he believed it was for him to take such a stand.

He "will not allow anyone or anything to stop him from getting the word home ... that he is the anti-busing messiah," Dixon wrote in the July 19, 1977, memo to Bayh.

As a young senator, Biden navigated complicated, often conflicting political currents on civil rights. Charles Harrity / AP file

Biden's allies say that doesn't sound like him at all.

After reviewing Dixon's version almost 42 years later, Doherty, who worked for Biden as a legislative assistant at the time and was put in touch with NBC News on Thursday by Biden's presidential campaign, said that he didn't remember the interactions, that he didn't have the power to decide who testified at hearings and that he didn't recall any time pressure to move the anti-busing bill through the committee.

"If that would have happened, I would have remembered it," he said of the exchanges Dixon reported to Bayh. And, specifically of Biden's remark, he added, "That would have been so out of character for him to say something like that."

Still, the full Judiciary Committee approved the bill and reported it to the Senate on Sept. 21 that year — on the timeline attributed to Biden — after an early August court ruling delayed the original start date for the busing plan. A year later, with a federal busing order set to take effect again just weeks before Biden would face voters, he and Roth proposed it as an amendment on the Senate floor, where it was fated to fail.

"I think Biden understood his re-election to his Senate seat was dependent upon establishing his anti-busing bona fides," said Brett Gadsden, author of the book "Between North and South: Delaware, Desegregation, and the Myth of American Sectionalism" and a professor at Northwestern University. "His position on civil rights and race and busing at the time, he's pretty mainstream."

Following days on the defensive over his 1970s-era Senate record on busing and other civil rights issues, Biden directly addressed his past in a speech Saturday in South Carolina, an early-voting state where black voters are critical. He invoked Barack Obama's decision to select him as vice president, saying he had been thoroughly "vetted" for the role.

And he said he's not the same person he was during his early years in the public eye.

"I’ve witnessed an incredible amount of change in this nation and I’ve worked to make that change happen," he said. "And yes — I’ve changed also ... I don't pretend to have gotten everything right. I don't pretend that none of my positions have changed. I've grown and I think it's good to be able to grow, to progress. Flawed and imperfect like everyone else, I made the best decisions I could at the moment those decisions had to be made."

But the questions swirling around his campaign on this front have never been fully addressed publicly. They are inextricably intertwined with half a century of social trends of progressivism and conservative backlash, local and national political electoral imperatives, and the arc of his own relentless ambition to ascend to the highest offices in the land. His civil rights record follows a track that has largely moved in synchronization with broader shifts in the Democratic Party and public opinion on equality in matters of race, gender and sexual orientation.

Most important, in this moment, it is one that has particular poignancy as he seeks the presidency, political historians say, because the United States is experiencing a period of political unrest in the aftermath of Obama's presidential tenure and in the midst of President Donald Trump's with echoes of the era in which Biden arrived on the national scene.

"Biden breaks in, in terms of where he is and who he is, right at this moment when the backlash politics pushes through," said Kevin Kruse, a Princeton University history professor and author of "White Flight: Atlanta and The Making of Modern Conservatism." "There really is a sense of 'it's gone too far.'"

Even now, Biden seems intent on turning questions of busing and his civil rights record on California Sen. Kamala Harris — hoping perhaps to take advantage of a pushback against her among his faithful.

The big question surrounding his candidacy may be whether he is deft enough to navigate the politics of backlash and consensus — to find a comfort zone between the liberals and conservatives — with as much electoral success as he did early in his career, even as he has to worry about winning a competitive primary for the first time.

The early signs for Biden are not good.

His role in the busing fights of the 1970s and the early 1980s re-entered the public consciousness following his remarks earlier this month about working with the late segregationist Sens. James Eastland, D-Miss., and Herman Talmadge, D-Ga. His fond recollections of working with senators he disagreed with on many principles and issues prompted an epic tongue-lashing from Harris, who is multiracial and who was bused as part of a desegregation plan, during last week's first Democratic presidential primary debate.

Despite his alignment with segregationists on busing and related matters during his early years in the Senate, Biden said Saturday that he was sorry he had suggested he'd found common ground with them.

"Now, was I wrong a few weeks ago? To somehow give the impression to people that I was praising those men who I successfully opposed, time and again," he said. "Yes, I was. I regret it, and I am sorry for any of the pain or misconception that may have caused anybody."

But, he said, he didn't want that to define his record on civil rights and that he would not allow his history of "fighting for civil rights and racial justice" to be "distorted or smeared."

The intricacies of Harris' argument have been complicated by ambiguity in her position on how federal power should be exercised to encourage or require busing now. But her ability to counter Biden's narrative of his own civil rights record, challenge the perception among many fellow Democrats that he is the best-equipped to go toe-to-toe with Trump in a fight, and connect voters to her personal story appeared to have an immediate and significant effect on both of their fortunes.

In one poll, taken by Quinnipiac, Harris has vaulted into a virtual tie with Biden, who has been the front-runner for the Democratic nomination since he entered the race in April. Other polls showed less dramatic movement for the same candidates, with Harris moving up and Biden down.

"Every candidate's record will (and should) be scrutinized in this race," Harris' press secretary, Ian Sams, tweeted Saturday morning. "It's a competition to become President of the United States. There are no free passes."

For Biden to recover, he will almost certainly have to find a way to discuss his record on civil rights in a way that doesn't portray him — the way Harris did — as so concerned about compromise that he became compromised on a matter of conscience for some Democratic primary voters.

His longtime allies, including those who were with him during his crusade against busing, say the matter was much simpler and more practical than that — it was about whether the remedy for segregated schools worked.

"Essentially, it wasn't going to help people who it was supposed to help," said Ted Kaufman, a longtime Biden aide who was appointed to fill his Senate seat temporarily when he won the vice presidency a little more than a decade ago. "If you look at Joe Biden, for his whole career he is the one who stands up for the people who don't have the power."

While opposition to busing is just a part of Biden's long record on civil rights, it was a prominent one that helped define his political identity as a consensus-seeking, third-way moderate — a Clinton-style Democrat two decades before Bill Clinton won the presidency — early in his career.

He came to the Senate as something of a political unicorn: a Democrat with a broadly pro-civil rights message elected in a border state at a time when even a handful of pro-civil rights votes could be career-killers for Democratic incumbents.

(Delaware was a former slave state, though it did not secede, and its state constitution long contained a provision requiring separate schools for white students and black students.)

He didn't find his political sweet spot on desegregation immediately. During his 1972 campaign against Republican incumbent Cale Boggs, Biden communicated so cautiously that the Wilmington Morning News wrote he was "for and against school busing," which allowed him to attract votes from black supporters of the method of integration and from white opponents. He likes to tell the story of announcing to segregationist Sen. John Stennis, D-Miss., that he had run for the Senate because of "civil rights" when he first arrived on Capitol Hill.

By late 1974, after he had enraged some of his white constituents by voting against Florida Republican Sen. Ed Gurney's amendment to strip courts of busing-order powers, Biden turned from a broadly reliable anti-busing voter into an anti-busing warrior. It took a flaying from some of those constituents at a town hall-style meeting to sear in the political reality.

"In the face of constant interruptions and heckling, he yielded the microphone," Gadsden wrote.

The meeting clearly left an impression on Biden, who swiftly found common cause with the arch-segregationists of the Senate. Not only would he go on to support anti-busing bills and amendments aimed at limiting the executive and judiciary branches' power in the area of busing, but Biden would also go further by proposing a 1975 amendment designed to prohibit the federal government from requiring schools to put black and white children in the same classrooms.

His amendment was a slightly watered-down version of one offered by segregationist Sen. Jesse Helms, R-N.C. Helms also wanted to prohibit the federal government from requiring schools to keep records on the racial makeup of classes, and Biden voted in support of that plan, too. Helms' amendment failed; Biden's was adopted but later dropped by a House-Senate conference committee.

At the time, critics assailed the Biden and Helms amendments as naked assaults on Title VI of the Civil Rights Act, which gives the federal government the power to withhold funding from institutions that discriminate based on race. Sen. Ed Brooke, R-Mass., the only black member of the Senate at the time, called the effort a "wolf in wolf's clothing."

Kaufman, who was given special floor privileges for the debate, said in an interview with NBC News that he had no recollection of the amendments or of the discussion.

"I have no memory" of it, he said.

But as the final vote on the Biden amendment showed — 50-43 in favor — the Delaware senator was hardly at the fringe of the Senate at the time.

Two days after the vote, the New York Times editorialized that "the principal tragedy of the Biden amendment, should it survive, is that it would signal a major crumbling of Federal determination to achieve equal justice" and it is "a real threat not only to the gains of the sixties, but to decency in this society."

Less than two weeks later, Biden told a Washington Post reporter that his amendment had been "a mistake" that was "not a planned thing." He said he "tried to emasculate Jesse Helms's amendment ... so I took his amendment and I amended it. ... I literally took a pencil and scratched out words in the amendment."

The debate on the floor that day did not hint at any lack of preparation on the part of the senators, and Biden voted against tabling — or killing — the Helms amendment.

"In the longer arc of white backlash against desegregation, he positioned himself between the signers of the Southern Manifesto ... and someone like Thurgood Marshall who is demanding that African Americans have their 14th Amendment rights," Gadsden said. For the segregationists, he explained, opposition to busing was actually a sign of having moved left from the days when they were trying to roll back civil rights. "It's in this moment that Biden, in responding to his constituents, finds common cause with them."

His mastery of backlash politics — frustrating as it was for liberals in the Senate — was evident in his reception at home. He was able to tamp down opposition among white Delaware voters, and he was in line with the wishes of some prominent black community activists who believed that busing for the purpose of integration created burdens rather than remedies.

"Oh Lord, busing was a mistake," said Bebe Coker, 83, who was a member of the Committee to Improve Education, which fought efforts to impose busing plans through the courts. "Our community was divided."

Of Biden, whom she plans to support in the 2020 primary, Coker said, "He's been more open to hearing the truth and understanding than most white people I know."

James Baker, who was elected to the Wilmington City Council the same year Biden won his Senate seat the first time and later served as mayor, said black schools in the area were in good shape before busing and that made Biden's constituency different than some others.

"Our students weren't getting secondhand books, outdated equipment," he said."For a large portion of the country, I think it was unique."

The Delaware-specific experience may help explain why Biden found himself making the kind of states' rights argument that long fronted Southern opposition to abolition, Reconstruction and integration — one that he echoed on the debate stage before fudging his record in pronouncing his commitment to federal power the next day.

"I did support federal action to address root causes of segregation in our schools and our communities, including taking on the banks and redlining and trying to change the way in which neighborhoods were segregated," Biden said at an event the day after the debate. "I've always supported using federal authority to overcome state-initiated segregation. In fact, I cast the deciding vote in 1974 against an amendment called the Gurney amendment, which would have banned the right of the federal courts to be able to use busing as a remedy."

And yet, it was the Gurney amendment that prompted the angry response to Biden at his town hall meeting in 1974 and led to the change in his fervor on busing.

After showing his anti-busing muscle to Delaware voters, including with his pushback against federal courts, Biden would go on to win re-election in 1978 with 58 percent of the vote — far more than the 50.5 percent he captured in his first race. He would never look back.

In his long tenure on the Judiciary Committee, including 17 years as chairman or top-ranking minority member, Biden would never be among the panel's most liberal members, such as Bayh and Sen. Edward M. Kennedy, D-Mass.

Still, there are any number of moments in that career that Biden supporters can point to as evidence of his progressivism on issues involving racial discrimination, including votes in favor of extending civil and voting rights laws, as well as fair credit and housing laws. His annual rating from the NAACP crested as high as 100 percent in line with the group's agenda but was typically less than that. In 1996, the year he voted for Clinton's overhaul of the welfare system, Biden's score was 60 percent from the NAACP.

Biden helped broker a compromise that led to a 25-year reauthorization of the Voting Rights Act in 1982, but he also worked closely for years with conservative Republicans, including longtime segregationist Sen. Strom Thurmond of South Carolina, on a package of anti-crime proposals that formed some of the basis for the 1994 Clinton crime law.

That measure, which ultimately included an assault weapons ban and collected votes in the House and the Senate from the broad middle of both parties, introduced harsher penalties for certain offenses and encouraged states to build more prisons. Critics say it helped usher in an era of mass incarceration — a charge that Biden denies — and that tough-on-crime policies of the era were an extension of the backlash to the Civil Rights Movement.

Trump is set to campaign in part on a criminal justice reform law designed to ease prison sentences, and he already has attacked Biden on the 1994 anti-crime law.

But Biden has cited Trump’s racial insensitivity — as exemplified in the president’s statement that there were “very fine people on both sides” of a white supremacist march in Charlottesville, Virginia, in 2017 — as the impetus for his presidential campaign.

"Very fine people on both sides?" Biden said in his launch video. "With those words, the president of the United States assigned a moral equivalence between those spreading hate and those with the courage to stand against it. And in that moment, I knew the threat to this nation was unlike any I had ever seen in my lifetime."