McCain will be stumping in Missouri today, a bellwether state that has voted against the national winner only once since 1904.
He will rally supporters in Kansas City, where Obama drew 75,000 Saturday, after speaking to a record crowd of 100,000 in St. Louis earlier that day.
McCain's vice presidential running mate, Gov. Sarah Palin, will be back on the attack in Colorado, but according to a new ABC News/Washington Post poll, she has become a liability for McCain.
The poll found that 52 percent of voters had less confidence in McCain after he chose Palin to be his vice presidential pick.
The biggest blow over the weekend was the decision by former Republican secretary of state and four-star Gen. Colin Powell to reject McCain's candidacy and endorse Obama instead.
Obama told NBC's "Today" show Monday that Powell "will have a role as one of my advisers." Whether Powell wants to take a formal role, Obama said, would be "something we'd have to discuss."
McCain and Palin will continue to hammer away at Obama, but that too has come with a cost.
The ABC News/Washington Post poll found that the constant barrage of attacks is hurting McCain, particularly when he targeted Obama's relationship with William Ayers, a founder of the 1960s radical anti-war group the Weather Underground.
The poll found that 60 percent of voters say Ayers is not a real issue.
The latest McCain tactic is to go after Obama's tax policies, describing them as "socialism."
And McCain can expect to face an avalanche of advertising in the last stretch of the race.
The Obama campaign announced over the weekend that the Democratic contender collected another $150 million from supporters during September.
McCain opted to accept federal funding limits for his campaign, restrcting him to $85 million in federal cash, of which he had $47 million remaining as of October 1, according to The Associated Press.
The toughest shot that McCain took was Powell's blistering critique of his choice of Palin for a running mate and the tone of the GOP campaign.
"It was very direct and very cutting,"ABC News' senior Washington correspondent George Stephanopoulos told "Good Morning America."
"This wasn't just an endorsement of Barack Obama. This was a rejection of Sen. McCain and President Bush," Stephanopoulos said.
Several things rankled Powell, who spoke to both candidates early in the summer and told them his endorsement would depend largely on their choice for vice president.
McCain, Stephanopoulos said, "took a direct hit by [Powell] saying he picked a vice president who wasn't ready to be president," Stephanopoulos said.
"Powell was also a little bit rankled that John McCain leaked that maybe he was considering Colin Powell for vice president," he said.
Obama is keeping up the pressure.
The Democratic nominee will be stumping Monday with former rival Sen. Hillary Clinton in Florida, another key state for McCain. It will be their first joint appearance at a rally since this summer.
The candidates have become a bit careless in some of their accusations as the campaigns have become more heated.
Fact Checking the Presidential Candidates
An ABC News fact check notes that Obama claimed in Virginia that McCain plans to gut Medicare by $882 billion to pay for his health care plan. McCain's camp insists that Obama is incorrect, that the savings will come from program changes, such as using generic drugs.
And McCain has charged that Obama's tax cuts will go to people who don't pay taxes, what McCain has called "welfare."
Obama's tax cuts will only go to people who work, although some people eligible for Obama's tax cut earn so little they don't pay income taxes. They do, however, pay payroll taxes.
What the Obama and McCain Tax Plans Mean to You An examination of the candidates' tax policies by the Tax Policy Center figured out what they could hold for you:
People who earn up to $18,725 a year would get a $65 tax cut from McCain, and a $567 cut from Obama.
Those who make between $37,000 and $66,000 would see an extra $608 under McCain's plan and $1,118 from Obama's plan.
If your income is between $66,000 and $110,00, McCain's proposal would mean $1,487 less in taxes, while Obama's would lighten the load by $1,264.
The biggest difference would be for those earning between $680,000 and $2.8 million. They would get a $109,214 tax break from McCain, but would pay an extra $121,689 under Obama's plan.