BOGOTA, COLOMBIA -- Colombia's national crisis over the spread of violence is complicating an already controversial effort by President Virgilio Barco to make the political system here more competitive.

The effort has involved the breaking of an old power-sharing agreement between the nation's two dominant parties and the establishment of a more conventional system in which the party out of power forms a clear-cut opposition.

As the security situation continues to deteriorate nationwide, pressure on Barco is growing to return to some kind of coalition of national forces, or what some are calling a "government of national salvation," to reassert legitimate authority.

A senior official said the president is still determined not to abandon his reform attempt. But following last month's killing of the attorney general by drug traffickers, Barco has begun meeting with leaders of the opposition Social Conservative Party in what is seen as a search for new political unity.

Three decades ago, when Colombia was last engulfed in widespread bloodshed, warring political parties secured peace by agreeing on a durable power-sharing formula. Introduced in 1958 as the National Front, the agreement ensured that Liberal and Conservative parties would share equally in Cabinet seats and other government posts, no matter which party won elections. The front became one of the longest-lasting political deals in Latin America.

It ended in 1986 when Barco, elected in a landslide, chose to run his administration strictly on Liberal Party terms, and the Conservatives declined the offer of a minority role in the Cabinet. Barco had said the old arrangement stifled democracy by the exclusion of other groups and by making it difficult to distinguish the policies of the two main parties. He said a healthier system would be one in which the winning party would govern and the losing party would become a genuine opposition party.

But Barco's effort to transform the way Colombia has been governed floundered from the start.

Politicians and diplomats here say some of the blame rests with the political parties. The Liberals are faulted for not taking advantage of their electoral majority to run the government forcefully and enact needed social reforms. The Conservatives are accused of obstructing the Congress and hassling the executive branch over the performance of various ministers, rather than offering clear alternatives to the government's program.

Another part of the problem, many say, has been Barco himself. Low-key in style, the Colombian leader has provided neither the inspiration nor decisiveness sought by the country in its time of crisis. While respected for his decency and administrative skills, the president, educated as an engineer in the United States, has appeared isolated from the public and sheltered by a small group of advisers.

"Barco's approach has been to go for long-term projects, not short-term rhetorical points in the press," said a political analyst. "But crisis after crisis has demanded immediate attention. Events seem to have overtaken Barco's ability to cope with them."

Presidential aides are vague about the aim of the new talks between Barco and political leaders, saying the objective is to reach accords on some basic issues, including new approaches to leftist guerrillas and cocaine traffickers and to relations with the United States.

The discussions have gotten off to a jolting start due to a sudden proposal by Barco -- surprising even leaders of his own party -- to seek a plebiscite for amending the constitution. What amendments the president has in mind have not been specified, nor is the plebiscite certain to happen, but the thrust of outlined changes would be to strengthen a number of government powers.

While the plebiscite initiative as well as recently announced antiterrorist measures have revealed a more energetic Barco, he may still be forced into some kind of coalition arrangement resembling the old National Front.

"The situation in Colombia is . . . highly abnormal," said Gabriel Melo Guevara, a Conservative senator, in a recent debate. "We cannot apply ideal formulas while we are vexed by such grave problems. The government-opposition scheme distracts and paralyzes. It is totally inadequate for the times."

Yet there is general agreement that some opening is desirable to incorporate leftist parties and defuse guerrilla violence.