Rahm Emanuel, President Biden’s nominee for ambassador to Japan, told a Senate committee on Wednesday that he would seize the offensive against China if confirmed — even as questions about his conduct as mayor of Chicago have put him on the defensive.

The Senate Foreign Affairs Committee took up Mr. Emanuel’s nomination seven years to the day after a white Chicago police officer murdered Laquan McDonald, a Black teenager, prompting weeks of protests and accusations of a cover-up.

“There’s not a day or a week that has gone by in the last seven years that I haven’t thought about this and thought about the what-ifs,” Mr. Emanuel said when asked about the anniversary.

Mr. Emanuel, 61, pointed to reforms he instituted after the killing. But he said he underestimated the distrust of his administration among Chicago’s Black residents.

“It is clear to me those changes were inadequate to the level of distrust,” he said. “They were on the best marginal. I thought I was addressing the issue, and I clearly missed the level of distrust and skepticism that existed, and that’s on me.”

Mr. Emanuel’s confirmation hearing represented an extraordinary collision of international affairs and a homegrown crisis, as participants alternated between a geopolitical discussion of the challenges posed by an ascendant Beijing and wrenching exchanges about police violence against Black people.

The McDonald case will probably not create a serious impediment to Mr. Emanuel’s confirmation, if the widespread praise for his nomination is any guide. Senior Democratic aides said they believed his appearance had bolstered his already solid chances of passing muster when the committee votes, as expected, in a few weeks.

Most Democrats mentioned the shooting briefly before moving on to foreign policy. And Mr. Emanuel received support from senators in both parties, including the committee’s ranking Republican, Senator Jim Risch of Idaho.

In fact, he was introduced to the committee by Senator Bill Hagerty, Republican of Tennessee and a former ambassador to Japan, who urged members of his party to back the former mayor.

“I intend to provide him with the bipartisan support that I was fortunate to receive from this committee,” Mr. Hagerty said.

Mr. Emanuel, who took a hard line against Beijing as President Barack Obama’s first chief of staff, cast the bilateral relationship, time and again, in the context of a larger conflict brewing between a group of countries known as the Quad — the United States, Japan, Australia and India — and China.

He began by delivering a stern warning to China’s leaders, citing military, foreign policy, public health and economic actions during the pandemic that he described as provocative.

“I think the world has learned a lot in Covid: We exposed some of our vulnerabilities, and I think China has been exposed for their venality,” Mr. Emanuel said.

“The region is desperate for America’s leadership,” he added.

His message was strikingly similar to the statements of R. Nicholas Burns, Mr. Biden’s nominee for ambassador to China, who appeared before the committee about an hour earlier.

Mr. Burns, who served in senior diplomatic posts for presidents in both parties, accused China of carrying out unfair trade practices, intimidating its neighbors — especially Taiwan — and “smothering” democracy in Hong Kong. He condemned the treatment of China’s ethnic Uyghur population as “genocide.”

But Mr. Burns, who went out of his way to praise President Donald J. Trump’s efforts to confront China on trade, offered a national pep talk of sorts, cautioning against overestimating China’s power and underselling American influence.

“The People’s Republic of China is not an Olympian power,” he said. “It’s a country of extraordinary strength, but it also has substantial weaknesses and challenges, demographically, economically, politically.”

China’s rise has particularly unnerved Japan, a nation with limited armed forces that also relies on the United States — which has some 50,000 troops based there — for protection against a bellicose North Korea. Japan has been wary of a shift in American political sentiment, fueled by Mr. Trump’s talk of freeloading allies and charging for U.S. military protection.

As ambassador, Mr. Emanuel would arrive in Japan at a time of political upheaval, including the surprise departure last year of Shinzo Abe, Japan’s longest-serving prime minister, because of ill health.

Mr. Abe’s successor is already about to be replaced with another unfamiliar face, leaving the Biden administration in need of fresh and reliable intelligence on the country’s leadership. The United States has not had a Senate-approved ambassador in Tokyo for more than two years.

From Tokyo’s perspective, Mr. Emanuel’s selection was a generally welcome one. In September, the English-language Japan Times noted that Mr. Emanuel is “known for his sharp tongue,” but wrote that he is close to Mr. Biden, “providing Tokyo with what could amount to a direct line to the White House.”

Mr. Emanuel is known for his abrasive personality, fierce partisanship and free-range profanity. He appeared to keep himself in check on Wednesday, but his infamous impatience poked through from time to time. He fidgeted in his chair as he listened to the senators opine, and thwacked his microphone to ensure it was working just before he starting reading his opening statement.

More than anything, the hearing showcased the fastidious preparation of a veteran Washington operator: Mr. Emanuel has spent years quietly developing relationships in both parties, and he worked his own nomination with determined focus (enlisting the former Obama press secretary Robert Gibbs to prep him for the questioning, according to a person with knowledge of the preparations).

He was especially careful to address the McDonald case in a conciliatory, if not entirely apologetic, way that emphasized his commitment to addressing underlying issues of racial inequality.

But questions about the McDonald case linger, centering on the delayed release of a police dashboard camera video showing the officer, Jason Van Dyke, firing his weapon 16 times at Mr. McDonald, 17, even as the young man lay in the street dying.

The video showed that Mr. McDonald was carrying a knife, walking and veering away from the officer when he was shot. It was not released for more than a year, and then only after a judge intervened. Mr. Emanuel’s critics have long accused him of foot-dragging.

After the video was made public, the city agreed to pay Mr. McDonald’s family a $5 million settlement, and the officer was eventually convicted of second-degree murder.

Mr. Emanuel told the committee he believed it would have been improper for him to intervene in the case. When “a politician unilaterally makes a decision in the middle of investigation, you politicize the investigation,” he said.

That did not entirely satisfy Senator Jeff Merkley, Democrat of Oregon, who pressed him for a more detailed explanation of his actions before Chairman Bob Menendez, a New Jersey Democrat, cut him off for exceeding his allotted time.

Several high-profile progressives, including Representatives Mondaire Jones and Alexandria Ocasio-Cortez, both of New York, and Cori Bush of Missouri, have called on Senate Democrats to reject his nomination over his role in the McDonald case.

“The man who helped cover up his murder is being considered for an ambassadorship. Rahm Emanuel has no business representing the United States,” Mr. Jones wrote on Twitter early Wednesday.