WASHINGTON — The worst-case scenario for President Donald Trump when former FBI Director James Comey testifies before the Senate this week might be if he refuses to answer questions about their meetings because they’re part of a criminal investigation.

That is thought to be one of the more likely results of at least some of Comey’s testimony.

Comey clearly has a story to tell. His not telling it may indicate that the president is in jeopardy from the criminal investigation, which is now run by former FBI Director Robert Mueller, who was appointed special counsel in the case.

Comey was overseeing the federal investigation into Russian measures to disrupt the 2016 election, and possible collusion in them by Trump’s campaign when the president fired him. If there was doubt about the reason for Comey’s dismissal, Trump cleared it by saying Russia investigation had weighed on him.

“I said to myself, I said ‘You know, this Russia thing with Trump and Russia is a made-up story, it’s an excuse by the Democrats for having lost an election that they should have won’,” Trump said on NBC.

Comey kept detailed notes on his meetings with the president. Comey associates said that at one of those meetings, according to Comey’s memo, Trump asked Comey for his loyalty. And Trump reportedly asked Comey to back off from investigating his first national security adviser, retired Army Lt. Gen. Michael Flynn.

But Comey’s not likely to reveal that in his testimony Thursday.

Comey has met with Mueller and discussed which topics are in-bounds, and which are best to avoid to protect Mueller’s investigation.

Paul Butler, a professor at Georgetown University Law School and a former federal prosecutor who specialized in public corruption cases, said it’s important not to read too much into questions Comey doesn’t answer. The timing, Butler said, is difficult.

“I doubt that Mueller is near a decision about whether to bring charges against anyone,” he said in an email response to questions. “He’s just putting his team together. If Comey is not forthcoming, it would just be a sign that Mueller is carefully protecting all of his options.”

Both Comey and Mueller, however, are aware of what can go wrong.

“There’s a history of congressional investigations messing up criminal prosecutions,” Butler said.

Members of both houses of Congress last week declined to predict what Comey will say. Based on what he’s said in the past, that’s a fool’s game.

But, they said, they are certainly curious. In a March hearing before the House Intelligence Committee, Comey for the first time announced the full scope of what had been known until then as the Russia investigation.

In that hearing, he said: “I have been authorized by the Department of Justice to confirm that the FBI, as part of our counterintelligence mission, is investigating the Russian government’s efforts to interfere in the 2016 presidential election and that includes investigating the nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia’s efforts.

“As with any counterintelligence investigation, this will also include an assessment of whether any crimes were committed.”

That statement turned the public perception of the investigation on its head.

Richard Painter, a professor at the University of Minnesota Law School, who was the chief ethics lawyer under President George W. Bush, said he had no idea what Comey might say. He did, however, have an idea of what would be important to hear, and that would be anything that hints that the Mueller investigation is looking at potential obstruction of justice.

“The key issue for obstruction of justice purposes is how much pressure the president was putting on Comey to drop the investigation, and whether he was threatened with being fired if he did not drop it,” Painter said in an email response to questions.