The fate of Todd Blanche’s nomination as attorney general remained uncertain on Wednesday after a rocky confirmation hearing in which a Republican senator raised serious questions about his role in creating a $1.8 billion fund for purported victims of Justice Department persecution.

The senator, John Cornyn of Texas, who was defeated by a Trump-backed opponent in a primary election, grilled Mr. Blanche about the fund and a related agreement granting President Trump and his family sweeping immunity from tax investigations.

Mr. Cornyn, a former judge, displayed text of the tax provision on a poster behind him and noted that Mr. Trump “has not agreed in writing” to nixing the fund.

After the hearing, he said he had not made up his mind. “I don’t have to make a decision now, so I’m not,” he said during a brief interview at the Capitol.

Even a single Republican “no” vote on the Judiciary Committee would block Mr. Blanche’s nomination from full consideration by the Senate, which could sink his confirmation. A second lame-duck Republican on the committee, Senator Thom Tillis of North Carolina, is also undecided but has said he is leaning toward voting “yes.”

The unusual two-part deal approved by Mr. Blanche, intended to resolve Mr. Trump’s lawsuit demanding at least $10 billion from the Internal Revenue Service over the leak of his tax returns, emerged as a major issue in a confirmation already clouded by questions about Mr. Blanche. As a top Justice Department official, he has had a role in protecting the president’s interests in the Jeffrey Epstein case and complying with Trump-ordered investigations of political opponents.

It is not clear when the committee will schedule a vote. First, Republicans must find a replacement for Senator Lindsey Graham, the South Carolina Republican whose death over the weekend cast a shadow on the proceedings.

Mr. Blanche, a former Trump defense lawyer, has served as acting attorney general since the president fired his predecessor, Pam Bondi, in April for not moving quickly enough to prosecute his perceived enemies.

Defying a president’s choice for such an important position would be an extraordinary gesture of defiance, even from a senator, like Mr. Cornyn, on his way out.

Justice Department officials have expressed confidence that Mr. Blanche will, ultimately, have the votes in the committee and the Senate at large. After he concluded his appearance, he walked back to a legislative conference room where the din of cheers and applause could be heard through an open door.

Mr. Blanche’s confirmation is somewhat symbolic. He could serve in an acting capacity for the remainder of Mr. Trump’s term. But the referendum on Mr. Blanche is in a broader sense one on the president’s vision of the department as a projection of his power and extension of his will.

It would be the capstone of a career that a few years ago seemed destined to be confined to the middle rungs of the New York-area legal community.

Before joining the president’s legal team a few years ago, Mr. Blanche earned a reputation as a highly competent federal prosecutor in the Manhattan U.S. attorney’s office. Democrats turned that perception against him.

Senator Adam B. Schiff, Democrat of California and a former federal prosecutor whom Mr. Trump has targeted for prosecution, accused Mr. Blanche of abandoning his ethical principles to serve his boss.

“What happened to the Todd Blanche who was a prosecutor in the Southern District of New York?” he asked. “What happened to the prosecutor people had respect for?”

Mr. Blanche, riled up, demanded the right to respond. “I am still here — I am the same exact person I was when I was a federal prosecutor,” he said, adding that his personal credo was “do the right thing, enforce the laws and put bad guys in jail.”

Mr. Blanche, typically a cautious and well-prepared congressional witness, made a significant unforced error that could fuel Democrats’ criticism of him as a Trump loyalist who has continued to act as the president’s personal lawyer in a post that requires a commitment to independence in the public interest.

Senator John Kennedy, a Louisiana Republican known for his folksy asides, asked Mr. Blanche what seemed to be a softball question: Did he consider the president to be his friend?

“I’m his lawyer,” Mr. Blanche replied, instantly correcting himself to add “was his lawyer.”

When Senator Chris Coons, a Delaware Democrat, asked Mr. Blanche whether Mr. Trump was eligible to run again in 2028, Mr. Blanche replied, “I don’t believe he is.”

Later, under questioning from a Democrat on the committee, Mr. Blanche maintained that he was unafraid to push back against Mr. Trump and prided himself on offering dispassionate legal counsel.

“Counsel does not mean yes-man,” said Mr. Blanche, craning forward in the witness chair in a dark navy suit.

Karoun Demirjian, Andrew Duehren and Carl Hulse contributed reporting.