Vice President Joe Biden's assertive debate effort righted the Democratic ship but the pressure is still on Barack Obama to come back from his own dismal performance if he wants a second term.

Obama's re-election hopes have hung in the balance since Mitt Romney knocked him about for 90 minutes on October 3 in the first presidential debate, with the incumbent seeming passive and putting up little resistance.

Biden stepped up to the plate on Thursday night needing a more robust performance if he was to slow the momentum of the Republican ticket, with Romney having seized the lead in several opinion polls.

"The most obvious takeaway was how much more combative it was," said Charles Franklin, politics professor and co-founder of pollster.com. "If the Democrats wanted Biden to be more aggressive, they certainly got that."

"It's hard to say if it has changed the momentum of the race dramatically, but I think it was a vigorous enough performance and it certainly did not reinforce the passivity that we saw from Obama last time."

Biden set about Romney's running mate Paul Ryan with relish, appearing incredulous at many of his statements and punctuating his replies with astonished exclamations like "Amazing," "Incredible," and "Malarkey."

He brought up Romney's "47 percent" remarks, when the Republican nominee appeared to write off almost half the electorate, but Ryan dodged the bullet well, reminding the electorate that Biden is prone to gaffes himself.

Unlike last week's presidential debate, much of Thursday's encounter centered on foreign policy and Biden called out Romney and Ryan for having no other answers despite their criticisms on Iran, Syria and Afghanistan.

"Are you going to go to war? Is that what you want to do now?" the vice president challenged his opponent, 27 years his junior, on Iran.

Dotty Lynch from the American University said that the Democrats she had spoken to were happy with Biden's performance.

"They are energized by it, they say he didn't let Ryan get away with anything, he stepped in when he heard something he wanted to refute."

But Ryan, a rising Republican star who is chairman of the House budget committee, didn't discredit himself either as he showed a good grasp of policy and hit back hard at his more experienced opponent.