Ryan definitely hurts Romney for a number of reasons:
1. Seniors - Especially in the MUST WIN state of Florida for Romney. There is no road to the whitehouse for Romney without Florida and Paul "I will gut SS and Medicare" Ryan is not going to be popular with the senior crowd. Florida was trending blue already, however, this seals the deal for Obama in Florida and essentially the Presidency.
2. Students - One of Romney's only hopes of winning the whitehouse is that the student vote would stay home. Students turned out in huge numbers in 2008 to vote for Obama...many of them are apathetic today. However, with Paul "I will cut Pell grants and student aid" Ryan, Romney has accomplished for Obama what Obama would not have been able to do....energize the student vote to get out and vote. The one thing students care most about is higher education funding. Romney/Ryan has just provided Obama with the ignition to light a flame under these voters.
3. Independents - Romney needs to the moderate/independent vote - Ryan hurts with the demographic. Ryan is too extremist fiscally for moderates/independents. Polling shows that moderates/independents are against the proposals in Ryans budget by large percentages. Socially, Ryan is off the deep end. Ryan will energize the teabaggers and the evangelicals, but Romney will need a huge shake of his Etch-A-Sketch to portray himself as a moderate/independent despite Ryan in order to bring in this group in any large numbers.
4. Women - Romney has essentially written off the female vote. Polls early in the campaign showed Romney having some inroads into this demographic...a demographic that Republicans have lost for the last 2 decades. Ryan destroys any hope of Romney making any inroads here....in fact, I suspect Romney's numbers in this demographic will fall dramatically over the next month, especially after the convention and Ryans extremist views are more well known.