In a rare return to the White House, Bill Clinton on Friday urged unhappy Democrats to unite behind President Barack Obama's embattled plan to avert a massive New Year's tax hike.

"I personally believe this is a good deal, and the best he could have gotten under the circumstances," the former president said as he and Obama paid a surprise visit to the White House briefing room after private talks.

Obama praised his guest -- "he presided over as good an economy as we've seen in our lifetimes" -- but quickly excused himself to go join his wife, First Lady Michelle Obama, at a holiday party.

Clinton, plainly relishing the spotlight, also pushed the US Senate to ratify a landmark nuclear arms control treaty with Russia and steeled Democrats routed in November elections for "a ferocious fight" with Republicans.

The former president, invited by Obama in the wake of that defeat, told insurgent Democrats he understood their anger at a compromise with Republicans to extend for two years a set of massive tax cuts due to expire January 1.

"However, the agreement taken as a whole is, I believe, the best bipartisan agreement we can reach to help the largest number of Americans and to maximize the chances that the economic recovery will accelerate and create more jobs and to minimize the chances that it will slip back," said Clinton.

Many White House allies have denounced Obama's decision to bow to Republican demands that rates stay low for the richest sliver of Americans and accept a rollback of the estate tax for all but a few thousand wealthy families.

Clinton, who highlighted how the plan extends unemployment benefits for 13 months, joked that he made "quite a bit of money now, so the position that the Republicans have urged will personally benefit me."

"And on its own, I wouldn't support it because I don't think that my tax cut is the most economically efficient way to get the economy going again," said the former president, who is vastly popular.

House Democrats have rejected the plan in its current form, but the blueprint appears set to clear a key US Senate procedural vote on Monday on its way to passage in the upper chamber.

Clinton also warmly endorsed Obama's call for the Senate to ratify the new Strategic Arms Reduction Treaty (START) this year, calling it "very important to the future of our national security."

"This is something that is profoundly important. This ought to be way beyond party. They worked very hard. They've worked out, in my opinion, the details, and I hope it will be ratified," said the former president.

Clinton suffered a mid-term election rout in 1994 but skillfully coasted to reelection in 1996 after tacking to the political center and making the most of Republican overreach -- a possible blueprint for Obama ahead of 2012 elections.

But Clinton declined to say what he told Obama: "I have a general rule, which is that whatever he asks me about my advice and whatever I say should become public only if he decides to make it public."

But, he added, "after the '94 election, I said the American people, in their infinite wisdom, they put us both in the same boat, so we're going to either row or sink. And I want us to row."

Asked whether he enjoyed his new role as advice-giver more than his eight years as decision-maker, Clinton said wryly: "Oh, I had quite a good time governing."

Earlier, Obama said in an interview recorded Thursday and broadcast Friday that he expected the US Congress to approve his tax compromise and that he hoped the Senate would ratify START before packing up for the holidays.

Many Democrats have denounced the compromise as an unnecessary giveaway to households that have prospered despite the painful recession and attacked the deal for swelling the already bloated US deficit and national debt.

"The agreement they reached is a bad deal for the American people. I think we can do better," Senator Bernie Sanders, an independent who often sides with Democrats, said during an unusual all-day speech on the empty Senate floor.