I would say that in this environment, if the Republicans win the senate this year and Clinton doesn't run, I would say that the Republicans are probably as favored as Democrats were in 2008 to win. It would be a very similar environment. In 2007, the Republican party was unpopular and lost most of the Federal and State races from the year before. Further, it seemed that their major policy, the Occupation of Iraq, was unpopular though Democrats couldn't do anything about it. To crown it off, the Republicans didn't have good candidates. If an even slight recession begins in 2015/2016, Bush or Walker will probably win with 53ish% of the vote. They probably won't have the same margins in congress that Obama had but somewhere between Bush and Obama at their heights.
So...60-70% chance of winning if they take the senate, 50/50 if they don't. If she does run, take 20% from those numbers.
While the difference between 30/70 and 50/50 may seem huge, you highlighted an interesting idea.
What you're saying is that Hillary running would only make a difference 1 in 5 times. While it may seem huge in terms of probability change, the result would remain the same 80% of the time.
Hillary's a big factor, but one of many variables.
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