Pressure on Obama in debate rematch

President vows to put up more of a fight after poor showing in first faceoff

With his hopes of a second term under threat, US President Barack Obama will seek to summon fresh energy on Tuesday to thwart Mitt Romney's momentum in their crucial second debate.

Republican Romney's assured performance in the first encounter two weeks ago in Denver, and Obama's own lifeless showing, dented the president's polling numbers, leaving the race effectively tied 21 days before the election.