They have promised "not to pitch one group against another for short term political or personal gain".
The Scottish National Party and Plaid Cymru have also signed the agreement, which was drawn up by the Commission for Racial Equality (CRE) after public concern that politicans were playing the race card.
The agreement says: "The right to free political expression must not be abused in the competition for popular votes by causing, or exploiting, prejudice and discrimination on the grounds of race or nationality."
And the leaders have accepted responsibility to "ensure that every alleged breach is properly investigated" and appropriate action taken.
Speaking in the Commons on Wednesday, Prime Minister Tony Blair welcomed the initiative and said he hoped candidates would "observe the spirit of it and not simply the letter".
CRE Chair Gurbux Singh called for "a thoroughgoing discussion of race issues in modern Britain".
He said: "Without effective debate on what the problems are and how solutions can be offered, we will not rid Britain of the discrimination that all political leaders have condemned.
"But we will only get that discussion when it is based on fact and not on assumptions, stereotypes or prejudices."
A spokesman for the race relations watchdog stressed that the Commission itself would not be adjudicating on public complaints.
No comment
Instead the aim would be to "bring the standards by which politicians wish to be judged out into the open".
The Commission would not be commenting on specific allegations of politicians using "inappropriate" language.
But it noted that in the past "politicians from all political parties" had been criticised by members of the public for debating in a way "likely to stir up and exploit prejudice".
Mr Hague was widely attacked for a speech he made earlier this month, when he said Britain under a second-term Labour government would become a "foreign land".
Foreign Secretary Robin Cook accused Mr Hague of "exploiting the worst instincts of fear and prejudice" - a charge the Conservatives strongly rejected.
And in April last year Liberal Democrat Home Affairs spokesman Simon Hughes reported both Labour and the Conservatives to the CRE over their handling of the asylum seekers debate.