Twenty-second Amendment to the United States Constitution
Some argue that the 22nd Amendment and 12th Amendment bar any two-term president from later serving as vice president as well as from succeeding to the presidency from any point in the presidential line of succession. Others contend that the original intent of the 12th Amendment concerns qualification for "service" (age, residence, and citizenship), while the 22nd Amendment, concerns qualifications for "election", and thus (strictly applying the text) a former two-term president is still eligible to "serve" as vice president (neither amendment restricts the number of times an individual can be elected to the vice presidency), and then succeed to the presidency to serve out the balance of the term (though prohibited from running for election to an additional term).
Joseph Estrada
Joseph Estrada stated in interviews that he would be willing to run for the opposition in the event that they are unable to unite behind a single candidate. Fr. Joaquin Bernas and Christian Monsod, members of the constitutional commission that drafted the 1987 Constitution, stated that the constitution clearly prohibits any elected president from seeking a second term at any point in time. Romulo Macalintal, election counsel of President Arroyo, clarified that the constitutional ban doesn't prevent Estrada from attaining the presidency in the event that he were to be elevated from the vice-presidency, for example.
However, Rufus Rodriquez, one of Estrada's lawyers, claims that the former president is within his rights to do so because the prohibition banning re-election only applies to the incumbent president.