I wouldn’t say that Coons would necessarily be a 1 term Senator. It’s probable that it wouldn’t be a wave election for the Republicans in 2014. And incumbency does have advantages.
But Coons is not Manchin at all. Coons has not tried to separate himself from Obama the way Manchin has. (Not predicting the WV race, just saying that Manchin is doing better than one would expect in a state that is 60% against Obama). Christine should be able to get everybody who is against Obama. And people who are against Obama are more likely to vote this time around.
I don’t think that Obama’s favorability is as low as 37% in Delaware though.
If Coons wins, he won't be up for reelection until 2016. Senators serve six-year terms.