The opposition candidate in next week's run-off in Liberia, Winston Tubman, has welcomed the resignation of the election head after fraud allegations.
Mr Tubman, a former UN diplomat, said his party would decide later on Monday whether it would now call off its planned boycott of the 8 November vote.
James Fromayan said he was stepping down because he did not want to be an "obstacle" to the run-off.
Mr Tubman is facing Nobel Peace laureate Ellen Johnson-Sirleaf.
She became Africa's first female elected head of state after the 2005 elections following the end of Liberia's 14-year civil war.
Mr Tubman's Congress for Democratic Change (CDC) had demanded Mr Fromayan's resignation, saying it would not take part in the run-off if he remained in his post.
It alleged widespread fraud in last month's first round.
"We presented photos of ballot boxes which had been tampered with by NEC [National Elections Commission] employees," CDC campaign manager George Solo told the AFP news agency.
"We also have the issue of tally sheets scratched out and their numbers changed. Old people wanted to vote for certain people but NEC workers were not open to assisting them.
Before stepping down, Mr Fromayan had rejected the fraud allegations.
"I am resigning to give way to peace. I do not want to be the obstacle to the holding of the run-off election," he said in his resignation letter.
His deputy Elizabeth Nelson takes his place.
These are the first post-war elections organised by Liberians - the previous poll was run by the UN, which still has some 8,000 peacekeepers in the country.
Mrs Sirleaf gained 44% against 32% for Mr Tubman. A candidate needs most than 50% for outright victory.
Mr Tubman's running mate is former footballer George Weah, who was defeated by Mrs Sirleaf in the 2005 poll.
Former rebel leader Prince Johnson, who came third with 12%, has urged his supporters to back Mrs Sirleaf.