The third indictment of Donald J. Trump has put Mike Pence at the center of an extraordinary moment in American politics.

He is at once Mr. Trump’s former vice president and his rival in the race for the Republican nomination. And as Mr. Trump is prosecuted for trying to reverse the 2020 election, Mr. Pence has emerged as a critical fact witness who took, as the indictment revealed, “contemporaneous notes” on the plot — and a man whom the president berated as “too honest” for his refusal to go along.

“Anyone who puts himself over the Constitution should never be president of the United States,” Mr. Pence said on Tuesday, setting himself apart from several other Republican candidates, who largely avoided criticizing Mr. Trump, even indirectly.

Mr. Pence has repeatedly suggested that Mr. Trump’s campaign to overturn the vote is disqualifying. But he has not said exactly how far he will go to prevent a second Trump turn in the White House, and whether those efforts would include testifying in court as a key witness for the prosecution. So far, he has stopped short of a broad-based condemnation of the president he served for four years.

Mr. Pence is involved in some of the most vivid scenes detailed in the indictment from the special counsel, Jack Smith, charging Mr. Trump with a conspiracy to defraud the United States and obstructing an official proceeding — the certification of the 2020 election.

At the very center were Mr. Trump’s efforts to pressure Mr. Pence to stop Mr. Biden from being certified as the winner in the Electoral College on Jan. 6, 2021. Mr. Pence had a ceremonial role that day, and Mr. Trump pushed him to exploit it to stay in power.

The pressure included a Christmas Day phone call, according to the indictment, in which Mr. Pence, an evangelical Christian, called Mr. Trump to say “Merry Christmas.” The president used the call as an opening to ask him to reject the electoral vote. Mr. Pence pushed back: “You know I don’t think I have the authority to change the outcome,” he said, according to the indictment.

They spoke again on New Year’s Day, when Mr. Pence again said that he had no constitutional authority to stop Mr. Biden’s ascent and that the effort was “improper,” according to the indictment.

“You’re too honest,” Mr. Trump told him.

Mr. Pence met with federal prosecutors this year and appears to have cooperated with them by describing the discussions he had with Mr. Trump between the election and Jan. 6. He has not said whether he would appear in court to testify against Mr. Trump, who made him a national figure when he selected Mr. Pence, then Indiana’s governor, as his running mate.

For months, Mr. Pence has maintained that “history will hold Donald Trump accountable” for his actions on Jan. 6. But he has avoided saying if the justice system should.

After repeating on Tuesday that Mr. Trump should never again be president, Mr. Pence added that he had not yet “reviewed” the indictment and reserved further comment for when he had. The rest of his statement mostly echoed what he has said before — with one notable omission. He did not condemn the charges, after saying as recently as 10 days ago that he “really” hoped none would come.

If the statement was a master class in political needle-threading, perhaps no politician has had a narrower needle to thread than Mr. Pence.

He spent more than four years as Mr. Trump’s running mate and then vice president, a period in which he was so loyal that a prominent vice-presidential historian called him the “sycophant in chief.” But then he defied Mr. Trump’s biggest demand, that he overturn the 2020 election in violation of the Constitution. Mr. Pence is still cautious when criticizing a man who retains the intense loyalty of the party’s base, but he also wants to beat him for the Republican nomination.

His own communications and actions are a crucial part of the evidence cited in the indictment. And notably, the indictment centers on an event — the storming of the Capitol — in which Trump supporters threatened Mr. Pence’s life.

“Hang Mike Pence!” the indictment quotes the crowd as saying. “Where is Pence? Bring him out!”

That makes this latest indictment against Mr. Trump a much deeper conundrum for Mr. Pence than the previous two, which concerned hush-money payments to a porn star and Mr. Trump’s retention of classified documents after he left office.

Mr. Pence called that first indictment “an outrage” and said the second one sent “a terrible message to the wider world that looks at America as a standard of not only democracy, but of justice.” After reading the details of the classified documents indictment, he acknowledged that the allegations were “very serious” and said he could not defend them, but still emphasized that he thought the decision to prosecute Mr. Trump was political.

In a CNN interview on July 23 — after the Justice Department sent Mr. Trump a target letter indicating that he was likely to be indicted in the election case, but before the indictment actually arrived — Mr. Pence said he “really” hoped the department would not file charges.

“Criminal charges have everything to do with intent, what the president’s state of mind was,” he said. “And I don’t honestly know what his intention was that day.”

“I’m not convinced that the president acting on bad advice of a group of crank lawyers that came into the White House in the days before Jan. 6 is actually criminal,” he said in another interview a few days earlier.