Mr. Romney was given a 28.0 percent chance of winning the Electoral College on Aug. 10, the day before he officially announced Mr. Ryan as his pick. The forecast then moved somewhat toward Mr. Romney after a series of improved polling in swing states for the newly minted Republican ticket, achieving a peak of 33.3 percent on Wednesday. It has since receded slightly to 30.6 percent, however, as Mr. Obama held leads in a number of swing state polls late last week.
These shifts could be consistent with a small vice-presidential “bounce” for Mr. Romney which has since faded — perhaps as less favorable stories for Republicans, like the comments on abortion and rape made by Representative Todd Akin of Missouri, have come to dominate the news cycle.
But these are only very minor differences — the model estimates that Mr. Romney gained a net of perhaps one percentage point in the popular vote after his selection of Mr. Ryan, and has lost perhaps half a percentage point since then. Changes of that magnitude could potentially be caused by statistical noise, as well as by real shifts of opinion.
What’s clearer is that Mr. Romney did not get as large a bounce in the polls from his vice-presidential pick as most past candidates have — a fact that can arguably be read as a bearish sign for him.