One might make a distinction between two approaches to argument analysis.  One approach looks at the assembly of facts asserted and wonders whether they ought to be candidates for assent.  The other approach, the rhetorical approach, measures an argument's effectiveness at producing beliefs, actions, policies, etc., independently of its mastery of "logic" or "facts" or anything else.  It's actually quite easy to determine whether arguments are successful on this approach, as you simply measure their effectiveness by the methods of quantitative sociology. 

One frequently relied upon rhetorical metric is the press (there are others–polling for instance).  Take the political press–find out what they're talking about, and you have a pretty good idea who is winning what argument (especially come election time).  This quick and dirty heuristic, however, grants the political press (a rather small group not representative of anything or elected by anyone) a lot of power in determining whether candidates are up to snuff.  And it also has the effect of making pettiness the center of the democratic process (hold the laughs).  To my mind, if you're going to measure the rhetorical effectiveness of "arguments," then at least pick a better measure than what the political press is yammering about. Here for instance is the view of a letter writer for the New York Times:

Recent articles help to clarify Barack Obama’s weakness not so much as a candidate but more basically as a potential functioning president.

Mr. Obama has been powerless to moderate the controversial views of his former pastor and, according to his campaign, Mr. Obama can now do nothing to deter the Rev. Jeremiah A. Wright Jr. from his divisive course.

Mr. Obama has chosen to separate himself from Mr. Wright, but even this decisive if belated action undercuts and perhaps illustrates the limitations of his claimed ability to bring us together. 

That fact that two things are getting talked about together is enough for this letter writer to suppose some kind of meaningful causal connection: after all, people wouldn't be talking about Obama and Wright if Obama were a better candidate, would they?  Obama ought to be able to shut the process down, and his failure to do that is evidence of his weakness as a candidate (the press can't shut up about it, after all). 

This reminds of an episode of Nightline on the Swift Boat business.  The moderator (or whatever you call him), asked entirely self-referential questions about the Swift Boat Veterans for Truth.  He reveled in the curiousity and the interest that people in the press, like him, were talking about the Swift Boat story.  He wasn't talking about whether it was true, but only what it meant.  And the fact itself that he was talking about it meant something, didn't it?  He wouldn't be talking about it if it weren't important.