This is highly entertaining...

And I've included a clip of Obama making the same mistake when announcing Biden as his running mate in 2008.

"Join me in welcoming the next President of the United States, Paul Ryan," Mr. Romney said as his running mate took the stage.

Mr. Ryan began speaking and, a few moments later, Mr. Romney jogged back to the podium to acknowledge his slip of the tongue.

“Every now and then I’m known to make a mistake. I did not make a mistake with this guy, but I can tell you that he’s going to be the next Vice President of the United States.”

Here's Romney's full speech:

Full text of Ryan's speech

Reuters - Paul Ryan Factbox

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Four years ago, Obama made the same mistake: