WASHINGTON — Senator James Lankford, Republican of Oklahoma, apologized on Thursday to Black constituents who were offended by his decision to join President Trump in trying to discredit the victory of President-elect Joseph R. Biden Jr., saying he had not realized the effort would be seen as a direct attack on the voting rights of people of color.

In a letter addressed to his “friends” in North Tulsa, which is predominantly Black, Mr. Lankford, who is white, acknowledged that his initial efforts to upend Mr. Biden’s victory — which he dropped in the immediate aftermath of the deadly assault on the Capitol by a pro-Trump mob — had “caused a firestorm of suspicion among many of my friends, particularly in Black communities around the state.”

“After decades of fighting for voting rights, many Black friends in Oklahoma saw this as a direct attack on their right to vote, for their vote to matter, and even a belief that their votes made an election in our country illegitimate,” he wrote in a letter first published by the news site Tulsa World and obtained by The New York Times. “I should have recognized how what I said and what I did could be interpreted by many of you. I deeply regret my blindness to that perception, and for that I am sorry.”

The letter offered the latest evidence of how the Capitol siege has rocked the Republican Party to its core, prompting some of Mr. Trump’s most loyal supporters to abandon him, alienating some of its crucial constituencies and setting off a painful period of soul-searching that could also have profound political consequences.

Mr. Lankford is facing re-election in 2022, and will soon have to decide whether to convict the president in an impeachment trial in which Mr. Trump faces a charge of “incitement of insurrection.”

While he did not offer a direct apology for questioning the legitimacy of votes, Mr. Lankford was among the handful of senators who withdrew his objection to counting some Electoral College votes cast for Mr. Biden after a throng of Mr. Trump’s supporters breached the Capitol complex. But it was a striking note of contrition, particularly as several of Mr. Lankford’s Republican colleagues who lodged the challenges, including Senator Josh Hawley of Missouri, continue to defiantly defend their efforts to throw out thousands of votes in Arizona and Pennsylvania.

The letter came amid calls from Black leaders for Mr. Lankford to resign from the 1921 Tulsa Race Massacre Centennial Commission, whose mission is to commemorate the racist massacre in the city’s Greenwood district, where a white mob destroyed an affluent Black neighborhood and its Black-owned businesses, and killed up to 300 residents.

Among the rioters who rampaged through the Capitol last week were members of white supremacist groups, and one man who carried a Confederate flag has been arrested.

Some Black leaders in Oklahoma said the senator’s note of regret betrayed a fundamental lack of understanding of how his actions had helped perpetuate racism.

“To use the words like any perceived racism — we’re in 2021 now,” said Greg Robinson II, an organizer and former candidate for Tulsa mayor who is among those who have called for Mr. Lankford and other Republicans to step down. “There has been generations upon generations of systemic racism that has been protected by the sort of white moderate rhetoric that we hear out of white politicians, especially white conservative Republicans.”

Mr. Lankford, a former Southern Baptist minister who directed the largest Christian youth camp before an inaugural run for office landed him in the House in 2011, has served in the Senate since 2014. Having burnished his credentials as a conservative Republican and deficit hawk, he muscled through a primary to win a special election and finish the term of former Senator Tom Coburn before a second victory in 2016.

In the Senate, Mr. Lankford has been a supporter of Mr. Trump, largely backing his policy initiatives and nominees even as he offered the occasional condemnation of the president’s vulgarity and personal attacks.

“I think most of us have a hard time with Donald Trump’s personality, but don’t have a problem with most of his policies,” said Frank Keating, a two-term governor of Oklahoma and veteran of multiple Republican administrations. “You can’t be much more conservative than James Lankford.”

But Mr. Lankford has also worked to build relationships with the Black community in Tulsa, speaking about the Tulsa massacre on the Senate floor and advocating the creation of a school curriculum to ensure that the 1921 massacre would be taught. When Mr. Trump announced plans to hold a campaign rally in Tulsa on Juneteenth, an annual holiday celebrated on June 19 that honors the end of slavery in the United States, Mr. Lankford was among the officials who successfully convinced the president it would be more respectful to hold the rally on a different day.

All five Oklahoman representatives and Mr. Lankford were among the more than 100 Republicans in both chambers seeking to invalidate the votes of tens of millions of voters in several states — many of them Black citizens living in Philadelphia, Detroit, Milwaukee and Atlanta — even as courts threw out baseless challenges by Mr. Trump and his allies about election malfeasance.

His involvement came as a shock to many on Capitol Hill and in Oklahoma, in part because he is regarded by Democrats as a rare, cooperative partner on voting rights. Some speculated privately that it had more to do with the fact that Mr. Lankford must face voters in two years than any actual concern he harbored about the integrity of the election.

“That result of that decision is bringing a hailstorm of criticism,” said a state senator, Kevin L. Matthews, founder and chairman of the 1921 commission. In an interview, he said he personally did not believe Mr. Lankford should resign from the commission, but that some members believed it was inconsistent with his drive to invalidate the election results. “There are a lot of people that feel like you can’t stand for both.”

Mr. Lankford and other Republicans had claimed that by challenging the election results, they were exercising their independence and acting in the interests of constituents who were demanding answers. In an interview the morning of Jan. 6, he sought to distinguish his argument from Mr. Trump’s false claims that the election could be overturned, saying he had been clear that there was no constitutional way to subvert the will of a majority of American voters.

“Everybody’s got their own motives in this, to be able to solve this,” he said. “For me, long term, we’ve got to be able to find a constitutional way to be able to resolve some of these issues.”

Less than four hours later, Mr. Lankford would be interrupted in his opening argument by the Senate’s sudden adjournment, as an aide whispered to him that the mob was inside the Capitol building.

In a secure location on Capitol Hill, Senator Joe Manchin III of West Virginia, recalled pleading with Mr. Lankford and Senator Steve Daines, Republican of Montana, to reverse course and support the counting of votes. The pair later released a joint statement calling on “the entire Congress to come together and vote to certify the election results,” and saying the lawlessness and chaos had caused them to change their minds.

“We disagree on a lot of things, and we have a lot of spirited debate in this room,” Mr. Lankford said that evening. “But we talk it out, and we honor each other — even in our disagreement.”

Reporting was contributed by Astead W. Herndon and Jim Rutenberg from New York, Reid J. Epstein and Luke Broadwater from Washington, and Mike Ives from Hong Kong.