The Foreign Office minister said the campaign to succeed Boris Johnson had been blighted by efforts to "discredit" fellow candidates that would provide "gifts to the opposition for years to come".
She has written to Liz Truss and Rishi Sunak urging them to sign up to a 'clean campaign charter' and commit to a contest that is "fair and focused on policy".
And she warned that the Conservative Party risked a "very grave situation" should it fail to "come together" once the new leader is elected.
Cannock Chase MP Ms Milling is one of a number of senior Tories to raise concerns over the toxic nature of the leadership race, which has been dogged from its onset by claims of clandestine briefings and dirty tricks.
In her letter, the former Tory chair said: "This leadership contest has been the most toxic contest we have seen to date.
"If we're going to have any chance of reuniting our party when it ends we have got to put a stop to the 'blue on blue' attacks that have been used to discredit fellow candidates and provided gifts to the opposition for years to come."
She urged the candidates and their teams to sign up to a charter that would see them committing to "stop the toxicity" and keep the contest "fair and clean" by focusing on policy.
"Our members deserve to hear what your plans are to deliver for the British people, how you're going to win the next election and how you're going to unite the party," Ms Milling said.
"To do that we have got to take the divisiveness out of the debate and show the party and the people that we are only focused on delivering for them.
"When this contest is over we have to come together as one team. If we don't we risk a very grave situation."
Last month a televised leadership debate was cancelled after both Ms Truss and Mr Sunak refused to take part amid claims the party's image was being damaged by in-fighting.