ST LOUIS, October 18, 2008 (AFP) - Addressing a sea of 100,000 supporters Saturday, Democrat Barack Obama fired back against White House rival John McCain on taxes and toxic campaign messages 17 days out from election day. Just over two weeks before the November 4 election, the Republican insisted that Obama's economic plan would drive up taxes and "kill" job creation as the United States weathers its worst financial crisis in decades. But the Democrat, who is riding high in national and state polls, said McCain was positing false arguments including via automated "robo-calls" to voters that portray Obama as a secret radical bent on subverting democracy.

"You guys have seen the ads. Some of you have received the phone calls," the Illinois senator told an enormous crowd in St Louis numbered by police at 100,000, his biggest yet in the United States.

Missouri voted for Republican President George W. Bush in both the last two elections, but Obama said the "winds of change" were blowing in the heartland state and across the nation.

He added: "With the economy in turmoil and the American Dream at risk, the American people don't want to hear politicians attack each other.

"You want to hear about how we're going to attack the challenges facing middle-class families each and every day."

At his own rally in North Carolina, another Republican state now very much in play, McCain once again invoked Ohio plumber Samuel "Joe" Wurzelbacher, who is now a hero of conservatives after challenging Obama on taxes.

"Joe's dream is to own a small business that will create jobs in his community, and the attacks on him are an attack on small businesses all over the country," McCain said as a harsh media spotlight turns on Wurzelbacher.

Obama says he would cut taxes for 95 percent of working Americans, but McCain asked his supporters in Concord: "How do you cut income taxes for 95 percent of Americans, when more than 40 percent pay no income taxes right now?"

The Obama campaign said McCain was distorting his plan, which would cut taxes for nearly every American who stumps up payroll taxes. Non-working people including those on welfare would not be covered.

But to raucus applause, the Arizona senator said the "Obama tax increase would come at the worst possible time for America."

Joe the Plumber and low taxes have become McCain's latest campaign motifs as the 72-year-old Republican vies to resuscitate his flagging White House hopes on the final stretch to the election.

Obama was ahead of McCain by 50 percent to 42 percent in Saturday's Gallup national tracking poll. In the Rasmussen tracking poll, the Democrat was up 50 percent to 45.

But Obama, 47, reiterated his message of recent days that supporters should not get "cocky."

"Democrats have a way of snatching defeat from the jaws from victory. You can't let up. You can't pay too much attention to the polls. We've got to keep running through that finish line," he said.

The Democrat noted McCain was describing his own plans for middle-class tax relief as government "welfare" that would take money from the rich to give to the poor.

"The only 'welfare' in this campaign is John McCain's plan to give another 200 billion dollars in tax cuts to the wealthiest corporations in America... that's who John McCain is fighting for," he said.

"But we can't afford four more years like the last eight. George Bush and John McCain are out of ideas, they are out of touch, and if you stand with me, in 17 days they will be out of time."

McCain is stepping up an offensive on Obama's ties to former 1960s radical William Ayers with "robo" phone calls in swing states that also accuse the Democrat of supporting the killing of babies born alive in abortions.

McCain's opponents unleashed robo-call smears when he was up against Bush for the Republican nomination in 2000, and even party colleagues such as Maine Senator Susan Collins say the tactic smacks of desperation this time.

"It's despicable, especially coming from John McCain," Obama campaign spokeswoman Linda Douglass told AFP.

"Voters around the country are offended and outraged at these calls. They want to hear about the issues," she said.

But the McCain campaign insisted the attack calls were rooted in fact and said Obama was hoodwinking voters over his past.