Barack Obama wheeled out a lean and punchy prototype for his second-term presidency, his purpose clarified by the voters who re-elected him a week ago.

In his first post-election press conference, Obama was a man in a hurry as he faces a year-end fiscal showdown with Republicans and anticipates the ebbing power that afflicts all second term presidents.

During his first four years, Obama, though Washington's dominant player, often came across as an almost passive observer of his own presidency.

Not so Wednesday as the president ditched his former professorial pose to hit the same point over and over again.

"I've got one mandate. I've got a mandate to help middle-class families and families that are working hard to try to get into the middle class," Obama said, in front of a flowing golden curtain in the East Room of the White House.

"That's what the American people said."

This was not the drained figure who trudged through the last year before putting in a listless convention speech and sleepwalking to disaster in his first debate against Republican foe Mitt Romney.

Since that debacle in Denver, Obama has pioneered a more concise style of argument, seeking to express focus and resolve.

With his job in peril, this no-nonsense Obama showed up when he directed the relief effort as superstorm Sandy pummelled New York and New Jersey.

The president used the murderous storm as a metaphor for the activist government he fought for in his campaign, and wants to see in his second term, which begins with his second inaugural at the end of January.

He described the federal relief effort as "aggressive and strong and fast and robust, and a lot of people have been helped because of it."

"That's a pretty good metaphor for how I want the federal government to operate generally," Obama added.

The president, who foes deride as arrogant, took pains to avoid hubris, despite being clearly energized by his election win.

"I'm more than familiar with all the literature about presidential overreach in second terms. We are very cautious about that," he said.

Obama may have been thinking about former president George W. Bush's ill-considered declaration that he had won a "mandate" despite squeaking to re-election in 2004.

As he spoke, Obama gripped the lectern bearing the presidential seal with his left hand, and gesticulated with his right -- a pose he often assumes at his most engaged.

He looked reporters clustered around him below a small stage directly in the eye, and pushed at the air with an outstretched hand, sparking flutters of shutters as news photographers raced to capture a freeze frame of action.

At times in his first term, and in some press conferences, Obama was windy and even boring, straying away from a cogent message.

But his opening statement topped out at 740 words -- compared to the 1,242-word lecture before his inaugural press conference as president in February 2009.

His message was simple and oft-repeated: Republicans must accept the verdict of voters and allow tax cuts for the rich, passed by Bush, to expire to permit deficit reductions that do not hammer the middle class.

Obama's demeanor suggested that unlike his first term, which began with massive expectations and an almost messianic zeal to forge "change," his second four years will be characterized by action, on a few big things.

But Washington awaits proof for the effectiveness of the new Obama tone.

If Obama is as punchy, passionate and nimble after another year waging war with Republicans in the gridlocked US capital, he will confound his critics.

The president signaled that after a hoped for deal to avert the "fiscal cliff" spending and debt showdown with Republican -- mostly on his terms -- he would proceed quickly to immigration reform.

"We need to seize the moment," Obama said.

Freed from the need to tread carefully with an election a few weeks away, Obama also responded furiously to attacks by Republicans on UN envoy Susan Rice and her role in the fallout of the attack on the US consulate in Benghazi.

"They should go after me," Obama declared, his eyes flashing fury.

Often in the past, Obama has raised expectations of action across multiple fronts, on sweeping and historic reform -- some of which passed, and some of which foundered, causing disappointment.

But the new Obama took care to lower expectations on one huge issue --global warming, following predictions he would take another shot at passing comprehensive legislation limiting carbon emissions.

He spoke vaguely about framing a coalition to tackle climate change and highlighted energy efficiency moves taken in his first term.

But he made clear that if tackling climate change distracted from his central purpose, he was not on board.

"If the message is somehow we're going to ignore jobs and growth simply to address climate change, I don't think anybody is going to go for that. I won't go for that," Obama said.