The Fatah faction has threatened jailed leader Marwan Barghouti with expulsion unless he drops out of the contest to become Palestinian president.
Marwan Barghouti is a popular figure on the Palestinian street
Two opinion polls published on Monday had him neck-and-neck with the official Fatah candidate Mahmoud Abbas.
But a third poll had Mr Abbas well ahead of his charismatic rival.
BBC correspondent Barbara Plett says Mr Barghouti's 11-hour registration turned it into a real contest, shattering the unity since the death of Yasser Arafat.
Fatah head Faraq Qaddumi confirmed that the popular West Bank leader would be thrown out of the movement unless he withdrew from the race.
"Marwan is one of our heroes," he said. "We hope he won't break the Palestinian consensus and destroy his reputation."
Mr Barghouti had by far the largest popularity rating apart from Mr Arafat before his death on 11 November.
He is currently serving five life sentences in an Israeli jail for ordering deadly attacks by militants.
Throughout his trial, Mr Barghouti refused to recognise the legitimacy of the Israeli court.
Palestinians are set to vote on 9 January.