NEW YORK - Virtually everything was going right for President Joe Biden as he opened the year.

His approval ratings were ticking up. Inflation was slowing. And as Democrats united behind his likely reelection campaign, Republicans were at war with themselves after a disappointing midterm season.

But on Thursday, Biden’s political outlook veered into more uncertain territory after Attorney General Merrick Garland appointed a special counsel to investigate the Democratic president’s handling of classified documents.

Democrats publicly and privately conceded that the stunning development was at best an unwelcome distraction at an inopportune time that muddies the case against Donald Trump. The Republican former president is facing a special counsel of his own and is under federal criminal investigation for his handling of classified documents and other potential transgressions.

There are major differences between the two cases. Most notably, there is no suggestion that Biden purposefully tried to prevent the documents discovered at his home or office from being turned over or that he was even aware of their presence. Trump, who is being probed for potentially obstructing investigators, also had far more classified documents in his possession.

But Thursday’s appointment of a special counsel nonetheless thrusts legal uncertainty over the sitting president and could revive debate among Democrats about the wisdom of him seeking a second term.

“No one’s going to say this is helpful,” veteran Democratic strategist James Carville said. “It’s pretty evident that’s not the case.”

As Democrats recoiled into a defensive posture, Trump’s would-be Republican rivals in 2024 acknowledged that the contours of the upcoming race had shifted.

Trump “is the luckiest man in American politics,” said John Bolton, who served as national security adviser under Trump and is considering a Republican White House bid. “This ought to be disqualifying to both of them.”

Thus begins a messy election season in which the current and former presidents of the United States are both under investigation by special counsels as they gear up for a potential rematch in 2024. Many voters in both parties were already calling for a new generation of leadership to emerge in the nascent presidential contest. Such calls are now growing louder.

“On many political fronts, Biden’s touted 2024 campaign is potentially vulnerable,” said Norman Solomon, a progressive Democrat who leads the so-called Don’t Run Joe campaign, which is already running television ads against Biden in key states. “Democrats and the country as a whole would be much better off this year and next if he’s not running for president.”

The 80-year-old president has already indicated he plans to seek a second term, but he has yet to make a final decision. His allies believe he is likely to make a formal announcement after the end of March.