Despite the political violence and intimidation McGee said the run-off should proceed as scheduled on June 27. "Anything less would give the Mugabe regime a victory they do not deserve," he said.
Tsvangirai beat Mugabe in the first round of elections in March, but did not garner enough votes to avoid the run-off.
Mugabe has ruled Zimbabwe since it's independence in 1980, but has resorted to increasingly desperate measure to stay in power in recent years.
McGee called on regional powers, in particular South Africa which has significant leverage with Zimbabwe but has been reluctant to use it, to send in election monitors to ensure as fair an outcome as possible and to protect the voters.
Secretary of State Condoleezza Rice said yesterday that the U.S. isn't currently considering imposing sanctions on Zimbabwe. However, McGee left the door open to other options pending the outcome of the elections.
The United States protested yesterday's harassment of its diplomats on a road north of Harare to Zimbabwe's ambassador in Washington and to Zimbabwe's delegation to a United Nations food conference in Rome.
ABC News' Dana Hughes contributed to this report from Nairobi, Kenya