President Trump’s reaction to former national security adviser Michael Flynn’s plea bargain has focused public attention once more on the issue at the heart of special counsel Robert Mueller’s investigation: potential presidential obstruction of justice.

This is nothing new. Since Trump fired FBI Director James Comey last May and cited the Russia probe, it has been evident that the president’s actions were creating a strong case for Mueller to conclude he had tried to obstruct the investigation.

But Mueller must ultimately resolve four questions to determine the extent of what may be the worst presidential misuse of power since Richard Nixon asked the FBI and CIA to cover up the Watergate break-in.

These are the four questions:

1. How far did Trump go to stop the probe of potential Russian involvement with his 2016 campaign?

He began at the outset of his presidency. On Jan. 27, Comey testified under oath, the president invited him for dinner and told him, “I need loyalty, I expect loyalty,” a pointed statement Trump later denied. A day earlier, acting Attorney General Sally Yates advised the White House that Flynn lied to Vice President Mike Pence and others (presumably including the FBI) about his contacts with the Russians, though Comey never said Trump explicitly mentioned that.

On Feb. 13, Trump fired Flynn — after The Washington Post reported he had lied to Pence. But after Flynn pleaded guilty last week to lying to the FBI, Trump tweeted, “I had to fire General Flynn because he lied to the Vice President and the FBI.”

Trump’s lawyer, John Dowd, claimed he wrote the tweet. But it was in the president’s name, an apparent acknowledgment he knew Flynn lied before his dinner with Comey and did nothing until it became public two weeks later.

On May 10, Trump fired Comey, blaming his handling of the 2016 investigation of Hillary Clinton’s email server. Two days later, Trump cited “this Russian thing” as a factor to NBC’s Lester Holt.

2. What was Trump’s motive?

Despite repeated denials by Trump and top campaign officials, a number have now acknowledged surreptitious contacts during and after the campaign with Russian officials.

It’s unclear if they intended these false statements to obscure potentially damaging contacts or something more sinister, perhaps stemming from Trump’s commercial interests and a long relationship with Russians funding them or, even more seriously, collusion with an active Russian effort to help him win the election.

The latest clue: In December, Flynn’s deputy, K.T. McFarland, wrote in an email obtained by The New York Times that the Obama administration’s strengthened sanctions could make it harder for Trump to ease tensions with Russia, “which has just thrown the U.S.A. election to him.”

3. Who shares Flynn’s guilt?

Flynn was charged with lying to the FBI. But others risk exposure for perjury. So far, at least a dozen people have made misleading statements, either to Congress, in government documents or to the public, including Trump, Pence and Attorney General Jeff Sessions. It’s too early to know if any will face legal charges.

4. What will be done about it?

That’s the ultimate question for Trump’s presidency.

Dowd told Axios the “president cannot obstruct justice because he is the chief law enforcement officer under (the Constitution’s Article II) and has every right to express his view of any case.” But presidential criminal liability remains a legal gray area. The one known prosecution venue is the constitutional provision calling for removal of the president “on impeachment for, and conviction of, treason, bribery or other high crimes and misdemeanors.”

“What we’re beginning to see is the putting together of a case of obstruction of justice,” Sen. Diane Feinstein, D-Calif., told NBC. If that’s Mueller’s conclusion, he could recommend Congress consider impeachment. After that, the outlook becomes murkier, given that pro-Trump Republicans run both House and Senate .

Still, it’s increasingly evident potential obstruction of justice is a principal focus of Mueller’s investigation. But not necessarily the only one.

Carl P. Leubsdorf is the former Washington bureau chief of the Dallas Morning News. Email at: