The first rule of vice presidential debates is to focus on the top of the ticket, as The Post's David Fahrenthold wrote in Thursday's newspaper. So how have Paul Ryan and Joe Biden measured up thus far?

With thirty minutes to go, both contenders largely have adhered to that rule, training their fire on Mitt Romney and on President Obama rather than on each other. One notable exception came during an exchange over looming defense cuts set to take effect in January 2013.

After Ryan argued that the Obama administration would enact the "devastating" cuts,  Biden shot back that the $500 billion trim was "part of a debt deal that [House Republicans] asked for." He pointed to Ryan's statement on the passage of last year's debt-ceiling deal, which set into motion the automatic cuts, paraphrasing Ryan as saying "we've been looking for this moment for a long time."

Ryan was quick to respond, noting that the "moment" he and other Republicans were looking for was a moment of bipartisan cooperation of the type that led to the debt deal.  The exchange was another example, however, of how Ryan has put on the defensive Thursday night, a shift from his running mate's aggressive performance last week in Denver.