I believe the extent to which the internet can be game changing in NATIONAL political campaigns is very much limited by the fact that all candidates execute extensive internet campaigns.
What the internet is good at is extending the tails of the guassian distribution of any given population. For example, it is now easier for each of us to find music better suited to our tastes. However, my own preference for music outside of mainstream pop will never redefine mainstream pop music. Pop music will remain uncomplicated and accessible to a broad audience; virtually by definition.
I feel like the same phenomenon is in play with presidential politics. There is some relatively small population of voters who lean toward Liberterian ideals. The internet has been instrumental in exposing these voters to Ron Paul. The same might be said for liberals and Dennis Kucinich. As a result, their support might be significantly greater than it would have been in the absense of the internet.
However, in the cases of the more moderate candidates with more mainstream appeal within a given party, I believe the internet has and will always have marginal impact. In fact, I actually believe that since all candidates have mostly effective internet strategies, the playing field is even and the only impact the internet can have will be if a candidate fails to use one.
Think about it...does a cellular phone really offer a competitive advantage in the business world in the United States? Of course not. Everyone has and uses a cell phone. There is no advantage to using one, but there is a huge disadvantage to not having one. I believe use of the internet in politics is comparable to the cell phone where national political campaigns that are focused on broad accessible concepts are concerned.
On the other hand, I do believe the internet can have a great impact in local elections with far lower turnouts. If a large enough minority can be mobilized for local school board, mayoral, state representative campaigns, I believe a considerbale impact is possible.