President Barack Obama is within reach of the 270 electoral votes needed to win a second term as Republican challenger Mitt Romney's path to victory is narrowing, five weeks from US election day.

To overtake Mr Obama, Mr Romney would need to quickly gain the upper hand in nearly all of the nine states where the two are competing the hardest.

Polls show the President with a steady lead in many of them as Mr Romney looks to shift the dynamics of the race, starting with their first debate tomorrow night in Denver, Colorado.

Mr Obama headed to Las Vegas on Sunday for a rally and planned to stay in Nevada, to prepare for the debate. Mr Romney remained in Boston, Massachusetts, going through preparations of his own.

Mr Romney's running mate, Paul Ryan, said: "In these kinds of races people focus near the end, and that's what's happening now."

Prominent Romney supporter, New Jersey Governor Chris Christie, said he expected Mr Romney's debate performance in Denver to shake up the campaign after a "tough couple of weeks".

He predicted Mr Romney would do "extraordinarily well" and that come Thursday morning, "this whole race is going to be turned upside down".

However, top Obama adviser David Plouffe said: "They expect to come out of this with the race fundamentally changed. Now, what does that mean?

"If it's going to fundamentally change, that means in seven or 10 days from now you'll see states like Ohio tied, the state of Iowa tied because that's what really matters here. So they have set the bar quite high."

One analysis shows Mr Obama would win at least 271 electoral votes today, with likely victories in Ohio and Iowa along with 19 other states and the District of Columbia. Mr Romney would win 23 states for a total of 206. A candidate must have at least 270 votes to win.