Mitt Romney (Left) with Ryan Paul
President Barack Obama is calling Mitt Romney's running mate the "ideological leader" of Republicans in Congress.
He says that ideology has led to gridlock, dysfunction and a top-down approach to growing the economy.
Addressing young supporters at a Chicago fundraiser, Obama called Wisconsin Republican Paul Ryan a decent, family man and welcomed him to the presidential race.
But Mr Obama says he fundamentally disagrees with Paul Ryan's vision for the country. He says Mr Ryan and his allies in Congress mistakenly believe that fewer regulations and tax breaks for the wealthy will help create jobs.
Mitt Romney announced on Saturday he had chosen Paul Ryan as his vice presidential candidate. Democrats pounced, arguing that Paul Ryan's budget-cutting agenda will harm seniors, students and the middle class.