Abdel Rahman al-Barr, a member of the Muslim Brotherhood Guidance Bureau, said bringing down presidential candidate Ahmed Shafiq is a "religious obligation."
National interest requires that Shafiq be knocked out of the race because he was part of the former regime's corruption, Barr added.
Barr described Shafiq as the enemy of the revolution who sought to suppress it as soon as it broke out.
In a telephone conversation with Mehwar satellite channel on Sunday afternoon, Barr said those who elected Shafiq cannot be described as sinful, though, because they have made an effort to choose the right candidate but may have picked the wrong one in the end.
Barr said patriotic Egyptians do not want the Mubarak regime to make a comeback.
According to unofficial election results, Shafiq came in second in the first round of the election with 23.9 percent of the votes, while Muslim Brotherhood candidate Mohamed Morsy ranked first with 24.8 percent of the votes.