Jim Messina was Barack Obama's campaign manager in 2012 and helped the Tories ahead of their 2015 general election victory.
He is currently campaigning for Hillary Clinton, and has spoken vehemently about the US political opposition.
Messina told Sky News:
We are in uncharted territory here with the Republicans.
Every single day they seem to make my day by saying crazier and crazier things.
He went on to suggest the GOP and the British Labour party have found themselves in similar boats, arguing the two biggest frontrunners (Donald Trump and Ted Cruz) have no chance of winning an election.
"The problem is every time [Trump] opens up his mouth about his vision he upsets voters," he said.
"I just saw a statistic that 47 per cent of Americans are very opposed to Donald Trump's candidacy. That's not a winning recipe in a general election."
Seemingly likening Trump and Cruz to Jeremy Corbyn and his shadow cabinet he said:
What you are seeing is a Republican Party who've gone off the rails a bit, much like Labour has in the UK, and are seemingly set on nominating people who can't win the general election.