DECEMBER 01, 02:31 EST

By LISA J. ADAMS
Associated Press Writer

MEXICO CITY (AP) - If Mexico's first opposition president in more than 71 years were to die in office, the rules of succession could send the country back into the hands of the ruling party he fought so hard to defeat.

Unlike the United States, Mexico has no vice president to assume office if its chief executive - a job being taken over by Vicente Fox at noon Friday - suffers a fatal heart attack or falls victim to an assassin's bullet.

Instead, the choice of a successor to Fox would be up the nation's Congress, a divided body in which, for the first time ever, no party holds a majority.

A team of Fox aides wants to change that. As part of a series of sweeping reforms affecting all sectors of government, they have proposed establishing a line of succession in the constitution.

The initiative would need approval by a two-thirds majority of Congress - which would be curbing its own power by passing it - and a majority of the country's 32 state legislatures.

It will be up to Fox to submit the initiative to Congress. His advisers will suggest that either the president or lawmakers decide who should take over in case of his death or removal.

The transition team also has proposed a change that would allow the president to be impeached.

While no party has a majority in Mexico's House or its Senate, the highest number of seats in both houses is still held by the Institutional Revolutionary Party, which Fox booted out of power in the July 2 election for the first time since it was founded in 1929. The PRI also governs 21 of the 32 states.

Mexico hasn't had a vice president since 1917, when the post-revolutionary Congress eliminated the position to protect the president. The thinking back then apparently was that anyone at the head of the line to succeed the president might be tempted to try to kill him.

``But now we're saying that our history is different, that there are no more reasons to argue that politicians would make an (assassination) attempt against the president of Mexico,'' said a spokesman for Fox's transition team who spoke on condition of anonymity.

Under Mexico's existing constitution, if a president leaves office for any reason in the first two years of his six-year term, Congress picks an interim successor by a majority vote - with two-thirds of the legislators forming a quorum. Congress must later call an election so that the people can choose a new president.

If a president leaves in the last four years of his term, Congress appoints a successor to serve out the rest of the term.

``In a Congress like ours right now, where there is no majority, can you imagine the war that could take place within the walls of that building?'' said historian Lorenzo Meyer. ``We have to think quickly of a different solution.''

Constitutional expert Ignacio Burgoa disagreed, saying even a divided Congress would eventually pick a president.

``We have a constitutional electoral system that is much better than the United States and many other countries,'' Burgoa said. ``We have never suffered a crisis like they are suffering in the United States.''