[he would be glad to] go back to the 1980s and assassinate Thatcher.

Said at a husting gathering at the GMB union's annual conference during the [[w:2010 Labour Party leadership election (UK)|2010 Labour Party leadership election (UK) cited in "John McDonnell says he would like to 'assassinate' Margaret Thatcher" The Telegraph (7 June 2010)

McDonnell withdrew from the campaign on 9 June 2010 because he could not gather sufficient nominations. In his September 2015 appearance on the BBC's Question Time he said: "It was an appalling joke. It's ended my career in standup, let’s put it that way, and I apologise for it as well."