Mr McGee said he had evidence of state-sponsored violence
Zimbabwean President Robert Mugabe has threatened to expel the US ambassador, accusing him of meddling in the country's political process.
"I am just waiting to see if he makes one more step wrong. He will get out," Mr Mugabe told a rally in Harare.
Earlier this month ambassador James McGee warned post-election violence in Zimbabwe was "spinning out of control".
Mr Mugabe was speaking as he launched his campaign for the presidential election run-off on 27 June.
He also said Zimbabweans who had fled recent anti-immigrant violence in South Africa would be given land if they returned to Zimbabwe.
"Our land is still there, even for youngsters, those who are in South Africa who wish to return to the country," Mr Mugabe told his Zanu-PF party supporters.
Earlier this month, Mr McGee told the BBC he had found evidence of "politically-inspired" violence against hundreds of people in Zimbabwe.
The diplomat warned the situation made it impossible for the second vote to be fair.
Mr Mugabe also noted that Mr McGee had publicly urged opposition Movement for Democratic Change (MDC) leader Morgan Tsvangirai to return to Zimbabwe to lead his embattled supporters in the run-off.
"As long as he carries on doing that, I will kick him out of the country," Mugabe said of Mr McGee, a Vietnam War veteran.
"I don't care if he fought in Vietnam. This is Zimbabwe, not an extension of America," he said.
According to Zimbabwe's election authorities, Mr Tsvangirai won the first round, but not by enough votes to avoid a second round.
He returned to Zimbabwe on Saturday after more than six weeks abroad.