MEXICO CITY — Last week, Alejandro Rojas and Layda Sansores were obscure ruling party legislators. Over the weekend, they became national heroes.

And this week, they may well become political martyrs--expelled from the powerful Institutional Revolutionary Party, known as the PRI, for a serious breach of discipline. They were the only PRI members in Congress to break ranks and vote against an unpopular increase in the sales tax.

Their stance has created a dilemma for the nation's long-ruling party, which is trying to project a more democratic image, both for itself and for Mexico. At the same time, the party is committed to supporting President Ernesto Zedillo's tough austerity measures aimed at stabilizing an economy shaken to its foundations by a crisis that has cut the value of the peso almost in half since December.

The most unpopular of those measures is a proposal to increase the sales tax to 15% from 10%. Business and grass-roots consumer groups have been equally opposed, for once uniting opposition parties on the left and right.

Even some PRI legislators who represent labor interests had threatened to vote against the measure, fueling speculation that Zedillo might become the first Mexican president in more than six decades to have a bill defeated in Congress.

But when the Chamber of Deputies voted Friday, the only nay vote from the PRI benches came from Rojas. In the Senate on Saturday, Sansores followed him, explaining, "I vote against a sales tax increase which has received an unprecedented rejection from a society battered by recent political and economic events."

By that time, the majority leader in the Chamber of Deputies, Humberto Roque, was already circulating a petition among the pro-labor legislators asking for Rojas' expulsion from the PRI.

The sales tax measure passed along party lines in both houses of Congress and will take effect April 1. But the dissent by Rojas and Sansores still caused a furor.

Rojas said he voted against the sales tax increase because it will harm his constituents, an argument that would be completely understandable in the U.S. Congress.

But in Mexico, Rojas and Sansores violated one of the most important unwritten rules of politics.

"The PRI is a party comprised of many diverse interests," one longtime member and former deputy said. "There is ample room for discussion inside the party. But once a decision is made, it is the responsibility of every member to support it."