With just over two weeks left before the November 4 election, Obama leads McCain in national opinion polls and in many of the battleground states that will be crucial to the the race.
The worst financial crisis in a generation has boosted Obama, whose calm demeanor has won over some voters anxious about the economy.
In hopes of overtaking Obama, McCain has focused heavily on the story of an Ohio man dubbed "Joe the Plumber," who raised questions about Obama's tax plans and became an invisible third member in the candidates' last televised debate this week.
"We learned that Senator Obama's economic goal is, as he told Joe, is to 'spread the wealth around,'" McCain told a rally in Concord, drawing loud boos from the crowd.
"Sen. Obama believes in redistributing the wealth, not in policies that grow our economy and create jobs and opportunities for all Americans."
In his third and final debate with Obama this week, McCain cited Joe Wurzelbacher as an example of someone who would be hurt under Obama's tax plan. Wurzelbacher said he intended to buy a plumbing business but believed he would receive a tax increase under Obama's economic plan.
Based on what Wurzelbacher has said publicly about his income, the Obama campaign said he would be eligible for a tax cut, not a tax increase, under the Democrat's proposal.
Obama says his tax plan would give a break to 95 percent of all workers and 98 percent of small business owners.
"John McCain is so out of touch with the struggles you are facing that he must be the first politician in history to call a tax cut for working people "welfare," the Democratic candidate said at the St. Louis rally.
"The only "welfare" in this campaign is John McCain's plan to give another $200 billion in tax cuts to the wealthiest corporations in America," Obama said.
The St. Louis event was Obama's largest in the United States, the campaign said. The only event he has held of comparable size was in July in Berlin's Tiergarten Park, where throngs of Germans gathered to hear him speak about transatlantic relations.
Obama, who dropped by a volunteer headquarters in Kansas City on Saturday to help work the phones, was to hold a rally in that city in the evening.
A Rasmussen poll on Friday showed Obama leading McCain by 6 percentage points in Missouri, 52 to 46 percent. The poll, with a margin of error of 4.5 percentage points, showed that McCain was favored by voters in the state during much of the summer.