The support of the former postal union leader, who is respected across the Labour party, will help the shadow home secretary as she seeks to position herself as the candidate best placed to beat Jeremy Corbyn.
The campaigns by Cooper and the shadow health secretary, Andy Burnham, have been thrown into turmoil by the success of Corbyn, who is speaking to packed meetings across the country. The fourth candidate, the shadow social care minister, Liz Kendall, is struggling to win support.
Johnson is one of the few Labour “big beasts” from the last government who is still left in the House of Commons after the retirement at the general election of Gordon Brown, Jack Straw and Alistair Darling.
Dubbed Tony Blair’s favourite trade union leader, Johnson is associated with the Blairite wing of the party. He might have been expected to back Kendall, who has been echoing much of the former prime minister’s warnings that Labour will only win power again if it reaches out to Middle Britain voters, who have supported the Tories in the last two general elections.
But Johnson’s backing for Cooper suggests that the Blairite wing of the party now believes that she is the candidate with some modernising credentials who is best placed to beat Corbyn.
Johnson, who was recently appointed by Labour’s interim leader, Harriet Harman, to serve as head of Labour’s pro-EU campaign group, has faced pressure to run for the leadership himself on numerous occasions in recent years.
There was a concerted attempt to persuade him to make himself available last year if Ed Miliband could have been persuaded to stand down. As a party loyalist Johnson declined the offer.
Johnson, who is a regular as #manontheleft on Andrew Neil’s This Week programme, has a personal charm and a fluency that has made him a popular figure. As deputy to former education secretary Charles Clarke, he said that they formed a double act in a charm offensive to persuade the Labour party to back university tuition fees. Johnson said Clarke did the offensive and he did the charm.
This article was written by Nicholas Watt Chief political correspondent, for theguardian.com on Tuesday 4th August 2015 10.19 Europe/Londonguardian.co.uk © Guardian News and Media Limited 2010