"If I do not succeeded ... I will resign" my position as a member of parliament, Talaat Sadat told a news conference in the capital, Cairo.
Sadat, from the small opposition al-Ahrar (The Free) party, said he could not sit in parliament and support "a president against my will, whomever that president might be", without mentioning Mubarak by name. "I am an Egyptian citizen, and I have my rights," he told the reporters.
Sadat's uncle - Mubarak's predecessor - was assassinated by in 1981 after making a peace deal with Israel.
Ten candidates, including Mubarak, have been accepted so far to run in the September 7 vote, with a final list due to be announced by Saturday.
The election has been touted as the first multi-candidate presidential vote in Egyptian history, but opponents have said the restrictions on becoming a candidate make it impossible for most people to stand.
Anwar al-Sadat, was Egypt's
president from 1970 to 1981
Furthermore, candidates must be party leaders, and the leadership of Sadat's party is the subject of a court dispute.
Sadat said that if he does not make the grade then he would come out in support of Ayman Nour, leader of the al-Ghad (the Future) party, one of Mubarak's fiercest and most controversial opponents.