Polls taken shortly after Mitt Romney selected Congressman Paul Ryan as his running mate show that the pick didn't generate a very positive response. A USAToday/Gallup poll found that only 39 percent American thought the choice was "excellent" or "pretty good" compared to 42 percent who thought it was "poor" or "fair." One reason for this is that few American actually know who Paul Ryan is, so it's hardly surprising that they weren't overwhelmed by the choice of someone they knew nothing about.
More importantly, the Ryan choice didn't give anyone a reason to rethink their presidential vote since there are few undecided voters in this election and there's nothing either demographically or ideologically different about Ryan compared to Mitt Romney. They are both conservative white men selected to lead a party composed mostly of conservative white men. If you are already leaning toward Barack Obama, the Ryan pick is hardly likely to change that. Nor is the Ryan pick likely to sway undecided voters. According to a recent analysis of by political scientists Larry Bartels and Lynn Vavreck, the undecided Democrats are mostly swayed by economic considerations, while the undecided Republicans are mostly moderate to liberal and unlikely to respond positively to the addition of another conservative to the GOP ticket.