Joe Biden's performance against Paul Ryan in the 2012 vice-presidential debate was the most discussed and debated political event of that week—praised by Democrats as the assertive, aggressive rebuttal they had desperately wanted from Barack Obama in the previous week's presidential debate, and criticized by Republicans as disrespectful and condescending.

Biden arrived with an evident mandate: shore up Democratic morale after Obama's widely panned Denver performance and disrupt Ryan's careful, policy-detailed persona. He delivered on both counts through a combination of direct argument, persistent interruption, and visible amusement at Republican claims he found implausible—most memorably the repeated, conspicuous smile and occasional laughter that dominated the visual narrative of the debate.

Ryan, the 42-year-old Wisconsin congressman and budget architect, performed competently. He was precise on policy detail, maintained composure under Biden's persistent challenges, and avoided significant errors. But he struggled to dislodge Biden's command of the room.

Instant polls produced split verdicts. CNN's sample gave the debate narrowly to Ryan on substance while acknowledging Biden's dominance of the physical performance. CBS's sample gave it to Biden. CNBC polls favored Ryan. The division tracked partisan lines more than genuine swing-voter sentiment.

The debate had a measurable effect on Democratic enthusiasm, which had sagged after Denver. Obama's campaign breathed easier; its vice president had demonstrated that the Republican ticket could be challenged.

For Ryan, it was a national introduction on the largest stage of his career to that point. He survived it credibly, even if Biden controlled the evening.