Let’s start this discussion with a brief backgrounder on the three branches of government. The Judiciary is headed by the Chief Justice. The Legislature is divided into the Senate (President of the Senate) and the House of Representatives (Speaker of the House). The executive department is headed by the President.
It’s not unusual for members of the Legislative Branch to cross over to the other branches of government. Chief Justice Hilario Davide was formerly an Assemblyman representing Cebu. President Estrada, among others, used to be a Senator. It’s also not unusual for members of the Judiciary to subsequently serve in the other branches of government. The late Marcelo B. Fernan became a Senator after his service as Chief Justice of the Supreme Court.
But it’s unusual for a President to become a member of Congress. No President, as far as I know, sought to be elected as a Senator. Certainly no President sought a position in the House of Representatives.
There are talks that Pres. Gloria Macapagal-Arroyo, after the end of her Presidency in 2010, would run for Congress also in 2010 — as a Representative of Pampanga. A former President serving as a congresswoman. There’s no legal prohibition, though I believe that it’s not proper. As to why, I can’t put my finger on it. Maybe you have an idea.