"The choice is made easy in part by Mr McCain's disappointing campaign [and] above all his irresponsible selection of a running mate who is not ready to be president," the article read.
The Los Angeles Times also gave its backing to Obama "without hesitation".
"As the presidential race draws to its conclusion, it is Obama's character and temperament that come to the fore. It is his steadiness. His maturity," the editorial read.
"These are qualities American leadership has sorely lacked for close to a decade."
It held sharp criticism for McCain, saying his campaign had rendered him "nearly unrecognisable".
"John McCain distinguished himself through much of the Bush presidency .... But the man known for his sense of honour and consistency has since announced that he wouldn't vote for his own immigration bill, and he redefined 'torture' in such a disingenuous way as to nearly embrace what he once abhorred," it read.
The Chicago Tribune came out in support of its home state senator on Friday as well, breaking with an anti-partisan policy that went back to the 19th century.
The newspaper editorials add momentum to Obama, who has been leading in national polls and pushing aggressively into traditionally Republican states.
Obama addressed a rally of more than 8,000 people on Friday in the state of Virginia, which has not voted for a Democratic presidential candidate in 44 years.
Urging supporters not to become complacent by his perceived lead, he said: "I want everybody running scared.
"Over the next 18 days, other than your family and your job, I want you to make a decision that there is nothing more important than bringing about this change that we need."
The latest nationwide Gallup poll of registered voters gave Obama a seven-point lead over John McCain, his Republican rival.
Polls by CNN and Time earlier in the week showed Obama ahead in several battleground states - up five points in Colorado, eight in Florida, three in Missouri and 10 points in Virginia.
Obama aides say his campaign plans to air advertisements in conservative West Virginia, where he lost the Democratic primary to Hilary Clinton by 41 per cent, and is considering pouring money into Kentucky.
Pro-Obama ads may also return to the airwaves in North Dakota and Georgia, they said.
Steve Lombardo, a Republican pollster in Washington, said: "It appears Obama is trying to build a mandate.
"Can McCain do anything to turn it around? Doubtful."
McCain, who campaigned in Florida on Friday, has been seen as weak on economic issues and is pinning his hopes for a comeback on a platform for lower taxes that has come to be embodied by Samuel "Joe the Plumber" Wurzelbacher, an Ohio tradesman.
Wurzelbacher, dubbed "Joe the Plumber" by McCain following his appearance at an Obama rally, was mentioned about two dozen times in the final presidential debate last Wednesday.
The two rivals appeared on the same stage on Thursday night for perhaps the last time in the campaign at a charity dinner in New York, making jokes about themselves and each other.
Obama told the audience that his own "greatest strength would be my
"Contrary to the rumours you have heard, I was not born in a manger. I was actually born on Krypton and sent here by my father, Jor-el, to save the planet Earth,'' he said, refering to Superman, the pop-culture character.
McCain said that Obama "is ready for any contingency".
"I'm told that at the first sign of a recovery, [Obama] will suspend his campaign and fly immediately to Washington to address the crisis,'' McCain said, poking fun at his own criticised move to temporarily halt campaigning a few weeks ago over the financial turmoil, as well as Obama's perceived political benefit due to the upheaval.