President Vicente Fox, angrily accusing Mexico's Congress of passing an "irresponsible" budget that could force tens of thousands of government employees to be laid off, made an unusual televised speech Thursday night and vowed to fight the spending plan.

"I have decided to legally challenge it, because my obligation is to defend you," Fox said, explaining why he was rejecting the $160 billion budget passed this week by the lower house of Congress.

Legal experts were divided Friday about whether the constitution empowers the president to veto the budget. Presidential spokesman Agustin Gutierrez Canet said Fox was considering a petition to the Supreme Court to clarify that question.

Such a showdown is rare in Mexico because for 71 years, until Fox's election in 2000, the Institutional Revolutionary Party, or PRI, controlled the government, and the federal budget was set by the PRI president and rubber-stamped by PRI legislators.

The PRI still dominates Congress and has consistently blocked Fox's most important legislative proposals, including changes in energy, labor and tax policies. Fox has blasted opposition legislators as obstructionist, but their feud seemed to reach new levels of nastiness over the budget issue. Gutierrez accused the PRI of a "personal vendetta" against Fox and of "wreaking havoc" with important programs to improve the PRI's chances in the 2006 presidential election.

The Treasury Department issued a statement Thursday saying the budget passed by Congress would require cutting about 30,000 jobs from the federal workforce, "practically paralyzing the federal government."

Fox's office issued statements Friday saying the PRI had steered millions of dollars for roads and other projects away from states governed by Fox's National Action Party, or PAN, to those headed by PRI officials.

Some analysts said Fox was simply outmaneuvered by his rivals in the PRI, especially Roberto Madrazo, the party's president and its most likely presidential candidate in 2006. "Fox was really taken to the cleaners on this," said political analyst Gabriel Guerra, adding that Fox compounded the political damage with a "complete overreaction" on national television. "Fox is just mad that they took the ball away from him."

President Vicente Fox, shown here at a recent event, went on television to denounce Congress's budget, which he said could force government layoffs.