This was a massive opportunity for Rand Paul and boy did he blow it.
I agree he ran a terrible campaign, but did he ever really have a chance? He looked decent in 2013-2014 as the GOP began to gradually move away from hawkishness, but after the rise of ISIS and multiple terror attacks, hawkishness was reinvigorated and he was basically DOA. On top of that, the rise of Bernie stole all the college students/internet activists/Redditors/young white men in general (his dad's main demographics) away from him, leaving him with basically nothing left.
I picked O'Malley and Walker, because both were largely hyped and had a real opportunity to become serious candidates / their party's nominee and they completely blew it. O'Malley was talked about as the major Hillary Clinton alternative, and he practically lived in Iowa prior to the election and only received about 0.5%. Realistically, he should've been the Hillary alternative and he allowed a 75 year old Democratic socialist from Vermont to steal that title from him. I love Bernie, and of course Bernie's strengths played into his rise (it wasn't only O'Malley that led to Bernie's rise) but I feel like O'Malley should've performed stronger.
I always thought O'Malley's flameout was predictable, for one simple reason: anyone who was against Hillary Clinton for being too right wing/neoliberal/warmongerish/whatever was not going to be afraid of a guy who called himself a socialist. If anything, it would be a positive (and indeed it was among that segment of voters.) O'Malley's constituency of "voters who think Hillary Clinton is too right wing but are also scared of socialism" is about as big as the percentage of the vote he received in Iowa. His campaign was bad though, but like Paul, even if he ran a good one I think he was doomed regardless. He was completely boxed out.