The democrats will badly struggle to replace Obama in this one. The energy and enthusiasm he brought to both 2008 and 2012 campaigns wont be there with Clinton (who will be nearly 70), I can't see her motivating young people or minorities the way Obama could, turnout will therefore be lower for the democrats.
If you remember in 08' Obama went to Germany and 250,000 people went to hear him speak, absolutely extraordinary for a guy to go to a foreign country and get that. Week after week in the US the guy was getting 100,000 people to hear him speak, 100,000 people in, Denver, Detroit everywhere in America, then he was the first person, beating LadyGaga and Bieber to 10 million twitter followers. In the cynical age of 21st century politics and how people view politicians normally, that's absolutely incredible.
Hilary is no Obama, the democrats are effectively attempting to replace possibly the greatest politician of all time (in terms of speeches, charisma, presentation etc), we saw what happened to democrats in the mid terms when Obama's name was off the ticket.
Many comments I'm reading in the NYT and Guardian suggest Obama will replace FDR as the greatest president of all time if gets the nuclear deal with Iran next week, the democrats are replacing a superstar effectively. The republicans will win this one as Obama is now off the ticket. The GOP wont increase their turnout, but the democrats will lose theirs in terms of young people and minorities.
Also I think there will be an anti-dynasty vote, Americans wont like the idea of another Bush/Clinton again in the white house, so if the Republicans avoid picking Bush and let the democrats pick Clinton, that will help them, Scott Walker looks like their best bet.
Both the Tories in the UK and Republicans are the natural part of government respectively in the US and the UK and are only put out of power when the left have a charismatic front man (e.g. JFK, Clinton, Obama, or Blair over here).