McCain told a noisy rally in Denver, "The answer to a slowing economy is not higher taxes, but that is exactly what is going to happen when the Democrats have total control in Washington."
Obama has proposed a tax increase on the 5 percent of taxpayers who make more than $250,000 a year and advocates tax cuts for those who make less.
The Arizona senator ridiculed the idea that any tax increases would be narrowly targeted and, at the same time, sought to link Obama to the unpopular Congress.
"Anytime you hear talk of a targeted tax increase, you might want to double-check the skill of the marksman - the U.S Congress has been known to fire wildly," McCain said. "America didn't become the greatest nation on earth by giving our money to the government to spread the wealth around."
The meltdown in financial markets and the national economic downturn have helped undermine McCain's standing in the polls nationally and in key battleground states because Obama is viewed more favorably than McCain on handling economic issues.
McCain continued to bash his rival as Obama left the campaign trail to visit the very ill grandmother who helped rear him.
"Sen. Obama says he's trying to soak the rich, but it's the middle class who are going to be put through the wringer, because a lot of his promised tax increase misses the target," said McCain. He said Obama's tax plans would boost taxes for half the nation's small businesses, a figure Obama disputes.
Speaking as the stock market plummeted again, McCain said he would focus on working families.
"I'm not going to spend $750 billion dollars of your money just bailing out the Wall Street bankers and brokers who got us into this mess," said McCain. "I'm going to make sure we take care of the working people who were devastated by the excesses of Wall Street and Washington."
Both McCain and Obama backed the $700 billion financial rescue package that President Bush signed into law.
McCain's plan to use half that money to have the government buy up troubled mortgages at full face value and then negotiate easier loan terms with the homeowners has been criticized by Democrats and a number of conservative Republicans on grounds it would not force profligate lenders to share in the losses from their bad loans.
Chanting protesters briefly interrupted McCain, but he dismissed them, saying voters "want us to stop shouting at each other." Police led the protesters, who were advocating rights for the disabled, from the arena.
McCain trails in most polls for Colorado's nine electoral votes, but he's scheduled three stops in the state and offered an optimistic prediction.
"This is going to be a tough state, but we're going to be up late and we're going to win here," McCain said.
At one point, he said "if I'm elected president," and quickly corrected himself to "when I'm elected president."
McCain had events in Colorado Springs and Durango, all focused on the economy, before he was to fly off to New Mexico, yet another battleground state. His schedule also contained a brief stop in Texas, then a visit to Iowa.