WASHINGTON (Reuters) - Barack Obama is revelling in presidential power and influence unseen in Washington for decades.

Barely 100 days in office, the U.S. president and his Democratic Party have firm control over the White House and Congress and the ability to push through ambitious plans.

Now, with the coming retirement of a Supreme Court justice clearing the way for him to appoint a successor, Obama already is assured a legacy at the top of all three branches of government -- executive, legislative and judicial.

On the corporate front, the federal government’s pumping of billions of dollars in bailout money into banks and auto companies has given Obama the power to force an overhaul in those industries, a remarkable intervention in capitalist industries by the state.

Americans are giving him leeway as well. His job approval ratings are well over 60 percent, giving him political capital to undertake big challenges.

His political opponents, the Republicans, are in disarray, reduced in numbers and engaged in an internal struggle over how to recover from devastating election losses in 2006 and last year.

Experts speak of Obama in the same league as such transformational presidents as Democrat Franklin Roosevelt, who led the United States through the Great Depression and World War Two, and Republican Ronald Reagan, who led the country to victory in the Cold War.

“I cannot in my memory remember a time when a president of the United States has had more influence,” said Democratic strategist Doug Schoen, who worked in the Clinton White House.

“Not only is it his moment, it is a level of influence and power for a president that is literally unprecedented from any time since the New Deal and Franklin Roosevelt. If he handles it right, it could be his century.”

STROKE OF LUCK

Obama is seeing evidence that with power comes the occasional stroke of luck.

A gift came in the party switch this week by Pennsylvania Senator Arlen Specter from the Republicans to the Democrats.

It could give the Democrats unfettered authority. If and when Minnesota Democrat Al Franken overcomes a recount battle and takes his Senate seat, Democrats will have a 60-vote supermajority in the 100-member Senate.

The announced retirement of Supreme Court Justice David Souter has given Obama the opportunity to put his imprint on the highest U.S. court and replace one of the court’s liberals with another liberal voice.

Although he will not be able to change the court’s balance of power from a conservative majority any time soon, he will be able to select a justice likely to remain on the bench long after Obama has left the White House.

How long will his luck last?

With the U.S. economy reeling and millions of Americans without jobs, Obama has his work cut out for him. He has a long list of challenges, and spoke of them at a news conference this week.

“If you could tell me right now that, when I walked into this office that the banks were humming, that autos were selling, and that all you had to worry about was Iraq, Afghanistan, North Korea, getting healthcare passed, figuring out how to deal with energy independence, deal with Iran, and a pandemic flu, I would take that deal,” Obama said.

Pollster John Zogby said that after his first 100 days, Obama now embarks on another weighty period in which Americans will want to see evidence that his $787 billion economic stimulus approved in February is working.

“I think by late June, they are going to have to start seeing some of these economic indicators stabilizing or at least some orange cones and hard hats out on the roads -- something that indicates either a stemming of the tide or some kind of progress,” Zogby said.

Those who keenly watch the poll numbers also point out that while Obama is personally popular, some of the items on his agenda are less so.

“I think he will always stay personally popular,” said Republican strategist Charlie Black, who was a senior adviser on Republican John McCain’s presidential campaign last year. “But when you test the individual policies, a majority will tell you we’re spending too much money.”

“On the policy proposals the jury is out as to how popular they will be,” he said.