WASHINGTON: In his high-velocity first 100 days, Barack Obama has sketched the outlines of a presidency of astounding ambition, which would remake the United States at home and transform its role abroad.

Yet the new president's agenda still faces tests of fire posed by a punishing economic crisis, the scheming of US allies and foes abroad and a poisoned political environment back home.

"It is clearly the most ambitious agenda at least since the 1960s," said Princeton University historian and political scientist Julian Zelizer.

Inheriting a crisis of a magnitude few recent precessors faced, Obama seems to have steadied America's nerves despite the wrenching financial blight.

He unleashed a huge government intervention in the economy, passed a historic 787-billion-dollar stimulus bill and now has high-stakes environmental and healthcare reforms on the launchpad.

Abroad, Obama swapped George W. Bush's swagger for a cupped ear, giving Russia respect it craves, reaching out to Muslims and vowing to drain decades of enmity with foes Cuba and Iran.

He previewed fundamental policy changes towards China, Mexico and Cuba, apologized to Europeans for past US "arrogance," mandated the closure of Guantanamo Bay, outlawed torture and ordered withdrawal from Iraq.

Obama doubled down in Afghanistan and Pakistan, ditched US denial on climate change and called for a world free of nuclear weapons.

Tom De Luca, professor of political science at Fordham University, said Obama's foreign policy was "breathtaking in its scope."

"I think it clearly illustrates the immense self confidence Obama has in himself and in his administration."

Obama, 47, once panned by rivals as inexperienced, slipped on the mantle of president with ease, in unflappable style.

Despite missteps on cabinet nominations and the fallout of Bush-era terror interrogations, his team has mostly dominated the political message.

The result: Obama is carrying the American people with him. Fifty-six percent in a USA Today/Gallup poll said he had done an excellent or good job.

A Fox News poll gave Obama a job approval rating of 62 percent, though like other surveys, found a sharp gap between Democrats and Republicans.

But talking about change is easy, effecting it is not: Obama admits America is a "big ocean liner -- it's not a speedboat; it doesn't turn around immediately."

He has yet to face a full-scale foreign crisis, and will face rising pressure to lift many families out of economic misery.

Many initiatives, including engaging Iran, battling the global crisis and subduing North Korea also depend on events and actors outside Obama's control.

For example, despite a euphoric welcome, Obama failed to crown a Europe trip this month with large numbers of new NATO troops for Afghanistan.

But the White House says results take time and exuded satisfaction after a Latin American summit last week.

"President Obama has on his first two foreign trips changed the image of America around the world through leadership and engagement that advances our national interests, makes us safer and more secure and stronger," his spokesman Robert Gibbs said.

Despite a fast start, eased by Democratic majorities in Congress and a demoralized Republican party, it is unclear whether Obama's political touch will endure on divisive issues like healthcare and climate change.

"He is going to have to deal with that ambitious agenda in the totally polarized domestic political environment," said Zelizer.

Debate still simmers over whether Obama is trying too much, too soon.

"Its not so much a question of whether hes bitten off more than he can chew, but whether it may be too much for Congress to chew and swallow this year," Bush's final press secretary Dana Perino said.

Obama aides counter that in dire times, the administration cannot afford not to attack all America's problems at once.

On the model of Franklin Roosevelt, whose frenetic first days in office in 1933 enshrined the 100 days formula, Obama adopted an experimental approach to rescuing shattered banks and frozen credit markets.

The stimulus bill, packed with spending on everything from high speed trains to rebuilding schools, also included tax cuts and was twinned by a simultaneous multi-billion dollar plan to aid the foreclosure-sapped mortgage market.

But Republicans warn Obama will bankrupt future Americans, eyeing his 3.5 trillion dollar budget and deficits even the White House says will reach 1.7 trillion dollars this year.