Mr Michael Ancram has resigned as Conservative Party chairman in order to join the contest to succeed Mr Hague as party leader.
In his resignation letter to Mr Hague, Mr Ancram said that over the last few days he had come under increasing pressure to join the leadership contest.
He added it had taken him a long while to finally decide to compete for the post.
Mr Ancram continued: "In these circumstances, it is imperative that without delay I stand down as party chairman, so that there can be no room for any suggestion that this important and over-arching party position is being undermined or abused by association with a particular leadership campaign."
Mr Ancram portrayed himself as a "unity" candidate as he launched his campaign.
He said: "I am not standing to fulfil long-held personal ambition, but to offer the Conservative Party the chance to choose the path of unity, to grasp the opportunity for reflection rather than hasty action and to renew our appeal to the people without tearing up our roots."
Mr Ancram, who arrived to announce his leadership bid in Victoria Gardens, adjacent to Parliament, flanked by wife Mary and daughters Claire and Jane, added: "I had not been planning a campaign for the Conservative leadership. As recently as last week I had not contemplated standing.
"I am putting my name forward today because I have been convinced by so many people both inside and outside Parliament that I cannot stand aside."
PA