Paul Ryan: Mitt Romney's choice for running mate sources tell CNN

(CNN) -- Mitt Romney's long-anticipated vice presidential announcement comes Saturday morning and sources indicated to CNN that the presumptive GOP nominee will name U.S. Rep. Paul Ryan of Wisconsin as his running mate.

Polls show Americans are largely unsure of Ryan, who chairs the House Budget Committee. A CNN/ORC International survey taken August 7-8 showed 54% of those polled saying they didn't know enough about Ryan to form an opinion. Twenty-seven percent said they viewed the Wisconsin congressman favorably and 19% viewed him unfavorably.

Among Republicans polled, Ryan's favorability was higher -- though 45% still said they didn't know enough about him to form an opinion. Ryan was viewed favorably by 49% of Republicans and unfavorably by 6%.

Twenty-seven percent of independents surveyed viewed Ryan favorably, compared to 22% who said they had an unfavorable opinion of him and 54% who were unsure.

When Republicans were asked in the who they'd like to see Romney choose as him running mate, 16% named Ryan. That placed him behind Florida Sen. Marco Rubio, who had the backing of 28% of Republicans, and tied with New Jersey Gov. Chris Christie.

The CNN poll was conducted by ORC International, with 1,010 adults nationwide, including 419 Republicans and independents who lean towards the GOP, questioned by telephone. The survey's overall sampling error is plus or minus three percentage points, with a sampling error of plus or minus five percentage points for questions only answered by Republicans and independents who lean towards the GOP.

While Ryan is unknown by many Americans, the seven-term congressman cruised to victory in 2010 with 68% of the vote in his Wisconsin district. The Badger State went for President Barack Obama in 2008 by nearly 15 percentage points, but the latest polling in Wisconsin shows a much tighter race this time around.

A Quinnipiac University/CBS News/New York Times poll showed 51% of likely Wisconsin voters backing Obama, compared to 45% who back Romney. The former Massachusetts governor won the Wisconsin GOP primary on April 3 with 44% of the vote.