Sen. Humberto Roque Villanueva, a senior figure in the Institutional Revolutionary Party publicly, called on PRI's President Roberto Madrazo to answer accusations that Madrazo conspired with Interior Secretary Santiago Creel to push the desafuero of López Obrador.

A newspaper recently made the allegations that Creel offered favors to the PRI, which holds the balance of power in Congress, if it votes against the mayor.

Creel ran against López Obrador in the 2000 Mexico City mayorality elections and lost. Madrazo beat López Obrador in Tabasco's 1994 gubernatorial elections, which were widely considered to be rigged.

Creel and Madrazo are leading contenders to be their parties' candidates in 2006 presidential election and may well face López Obrador of the Party of the Democratic Revolution (PRD); if the desafuero does not disqualify him from running.

Opinion polls show that if he runs, he wins, and if it were not for the desafuero over a seemingly trivial land dispute, the hugely popular mayor would be unstoppable.

The mayor has accused Creel of plotting to keep him from the presidency.

Sen. Roque Villanueva said: "If Madrazo doesn't personally answer the allegations, we will be shrouded in speculation, which is not good for the PRI."

The red hot dispute, which has ignited a national controversy and sent protestors to the streets, is expected to be at least partially resolved Friday, when a congressional committee will recommend to the full Congress whether López Obrador should face prosecution.

The desafuero process is over his alleged ignoring of a court order to stop building an access road.

López Obrador built his support base on heavy spending on social programs and construction projects. Critics compare his increasing the capital's debt to the actions of Venezuelan President Hugo Chávez. The United States government is hostile to Chávez, and the CIA recently warned that Mexico could become unstable as it goes into presidential elections.

The desafuero process has not bolstered many Mexicans' belief in their democratic system.

To them, the seemingly trumped up charges against the mayor are infuriating, and they see it as a plot to stop López Obrador becoming president.

"This government promised change. But instead they behave like PRI," said Juan Sánchez, 32, a student protesting in Mexico City.

The federal Attorney General's Office, which started the scandal last May when it asked Congress to remove his immunity, defended itself in full-page newspaper advertisements this week. The Supreme Court did the same the next day, using the advertisements to explain the legal steps to date.

Mexico City's chief prosecutor, Bernardo Batiz, on Monday accused federal officials of pursuing the mayor "with the clear intention of disqualifying him as a politician." Attorney General Rafael Macedo de la Concha replied that his department is focused on "justice, not politics."

As the nation awaits Friday the mudslinging and marching continue.

"We'll keep protesting until he's cleared of all charges and can run fairly for the presidency," said Ana María Martínez, a farmer from the State of Mexico.