Drew Angerer/The New York Times

Former President Bill Clinton held a remarkable, spur-of-the-moment news conference at a White House podium on Friday to announce his backing for the tax compromise President Obama reached with Republicans this week.

Mr. Clinton and Mr. Obama turned up in the White House briefing room after meeting privately together in the Oval Office. Mr. Obama introduced the 42nd president and then stood by as the one-time occupant of the White House offered his thoughts.

“I have reviewed this agreement that the president reached with the Republican leaders,” Mr. Clinton told reporters who assembled quickly for the hastily called remarks. “The agreement taken as a whole is, I believe, the best bipartisan agreement we can reach to help the most Americans.”

The image on the television screen – of Mr. Clinton at the White House, behind the official podium – had a time-warp quality to it.

The president stood by Mr. Clinton’s side for several minutes as Mr. Clinton held court in front of the White House logo that often hovered behind him a decade ago.

But after Mr. Clinton began taking questions, the current president excused himself, saying that his wife, Michelle, expected Mr. Obama’s presence at one of the many holiday parties that presidents host during the month of December.

“I’ve been keeping the first lady waiting,” Mr. Obama said, excusing himself.

“I don’t want to make her mad,” Mr. Clinton said. “Please go.”

And with that, Mr. Obama departed, leaving Mr. Clinton to continue his extended conversation with the media. The image on the television screen – of Mr. Clinton at the White House, behind the official podium – had a time-warp quality to it.

The decision to ask Mr. Clinton to the White House, and then to have him make a public statement, reflects the desire by the White House to counter the anger among liberal Democrats, who have accused Mr. Obama of caving too quickly on the tax deal.

For the past several days, the White House has been touting the support of numerous Democrats, from the mayor of Kokomo, Ind., to numerous governors and senators. But Mr. Clinton is clearly the biggest gun in that arsenal that Mr. Obama could deploy.

“In my opinion, this is a good bill and I hope that my fellow Democrats will support it,” Mr. Clinton said. “We all see this differently. But I really believe this will be a significant net plus for the country.”

Mr. Clinton went on for at least 20 minutes, moving at one point beyond the tax debate and offering his opinion on the administration’s new arms control treaty with Russia and the ongoing crisis in Haiti.