(SAUL LOEB/AFP/GettyImages)

CHICAGO - Just over 24 hours after Rep. Paul Ryan was tapped for the vice presidential slot on the Republican ticket, President Obama today welcomed the Wisconsin congressman to the race, branding him the "ideological leader of Republicans in Congress."

Addressing donors at a campaign fundraiser on the south side of Chicago, Obama slammed his rivals' belief in "top-down economics" as a solution to the nation's economic woes, insisting the approach has been tried and failed.

"This kind of top-down economics is central to Gov. [Mitt] Romney and it is central to his running mate," Obama told roughly 1,000 donors huddled inside the Bridgeport Arts Center.

"Just yesterday, my opponent chose his running mate, the ideological leader of Republicans in Congress, Mr. Paul Ryan. I want to congratulate Mr. Ryan. I know him. I welcome him to the race," Obama said.

Some members of the crowd began booing at the mention of Ryan, but Obama cut them off.

"He is a decent man, he is a family man, he is an articulate spokesman for Governor Romney's vision, but it is a vision that I fundamentally disagree with," he said.

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The remarks were the first time Obama publicly acknowledged Romney's running mate since the announcement Saturday morning in Norfolk, Va. Vice President Joe Biden called Ryan Saturday to welcome him to the race, the Obama campaign said.

Obama and Ryan will face off on the campaign trail for the first time Monday with dueling events scheduled in Iowa, a key Midwest battleground. The president begins a three-day, seven-stop bus tour through the state, while Ryan plans to "drop by" the Iowa State Fair in Des Moines.

Romney campaign spokesman Ryan Williams said Republicans are girding for a fight.

"We all know that President Obama will continue to run a fear and smear campaign because his policies have failed and he has no vision for the future of our country," Williams said in a statement. "Mitt Romney and Paul Ryan bring new ideas about creating jobs and controlling the budget. President Obama's campaign has accused Mitt Romney of committing crimes and killing people, and now they have turned their smear campaign to Paul Ryan. They have lost all credibility."

Obama is in Chicago for a whirlwind day of five re-election fundraisers expected to raise at least $6.4 million combined for the 2012 race, according to estimates provided by the Obama campaign.

Obama has spent a record-smashing amount of time fundraising as an incumbent, attending more than 200 events in his first term. President George W. Bush had attended just 88 fundraisers at this point in his first term.