US President Barack Obama has dubbed Mitt Romney's running mate Paul Ryan yesterday as the "ideological leader" of congressional Republicans.
Mr Obama said that Mr Ryan is bent on imposing "top-down economics" on middle-class Americans.
Mr Ryan is viewed as the architect of a budget plan that would make deep cuts to the US social safety net.
He will be portrayed by the Obama campaign as a threat to the elderly and the poor.
President Obama said: "He's a decent man. He is a family man. He is an articulate spokesman for Governor Romney's vision. But its a vision that I fundamentally disagree with."
He spoke over boos from some in the audience at the mention of the Wisconsin congressman's name.
The remarks were Mr Obama's first comments about Mr Romney's selection of the conservative as his running mate, which has injected new energy into the Republican campaign.
Democrats will try to scare independents, senior citizens, and liberal voters over the Mr Ryan's controversial cost-cutting proposals, such as reforms to Medicare.
Mr Ryan is an economics expert recognised even by Democrats as a budget heavyweight.
Meanwhile, the campaign team for Mr Romney, the Republican presidential candidate, reviewed several years of tax returns as part of the vice presidential search process.
But Mr Romney, a former private equity executive with an estimated net worth of up to $250m (€203m) has refused to publicly release more than two years of tax returns.
The secrecy has led Democrats to say he has something to hide about his vast wealth.
