South America's two major exceptions to the pattern of military rule, Colombia and Venezuela, do not take their luck for granted.

Their governments borrow elements that North Americans would think alien to democracy, including strong secret police and occasional formal states of siege. The two countries are relatively new at the democratic game, and it is frequently rumored that they are about to topple into uniform.

Both suffer guerrilla attacks, both have a large poverty-stricken section of the population and both undergo occasional crippling strikes and bloody student demonstrations.The ingredients for military takeover would seem to be there, but both countries are doggedly planning national elections in 1978. Indeed, the campaigns have more to less started already.

"Rational use of our oil income is the last chance for Venezuelan democracy," said President Carlos Andres Perez when he took office in 1973. The oil money that provides 86 per cent of the nation's $10 billion annual income is financing massive public-works projects, industrial development and education plans to bring the population together and into full participation.

Even so, the elections in 1958 that ended a brutal dictatorship were only the second since the founding of the country.

President Perez has done his time in jail and exile, running clandestine operations into the country he was later to head. As minister of the interior in the early 1960s, he ran a tough secret police that went after Cuban-inspired guerrilas. His enemies treme rightist, and it was rightist support that made him president.

But Perez has steered clear of the obsession with national security that characterizes the military regimes south of Venezuela. The leftist Movement Toward Socialism (MAS) party, often accused of Communist ideas, may make a strong showing in 1978. Perez has complained that MAS publications slander him and has jailed members of Parliament suspected of guerrilla involvement, but he also quickly fired the head of his secret police unit when a suspected guerrilla died under interrogation.

In Colombia, the pressures for a national security orientation seem to be stronger. Resistance from the old families to land reform, modern taxation and public-sector investment has stalled President Alfonso Lopez Michelsen's attempts at reform.

The resulting unrest, strikes and demonstrations led him in July 1975 to reimpose the state of siege that had been more or less a way of life before he took office. Now the highly vocal, if splintered, left calls his system a "paper democracy," pointing to the high abstention rate in recent off-year elections for which the state of siege was briefly lifted.

At the same time, the army has been battling myriad guerrilla groups, occasionally occupying turbulent campuses and breaking up street riots with tear gas. Rampant street crime and drug traffic have given Bogota an international black eye, especially with tourists, while administrative corruption and paralyzing red tape seem to strangle all attempts at cleaning up. By other Latin American countries' definitions, the situation would appear to cry for imposition of military order.

Yet the Liberal and Conservative parties have alternated power in overthrows and elections of varying degrees of farce since Colombia's founding. When the system broke down, the strong divisions that still persist led to a bloody undeclared civil war between the two parties from 1948 to 1957, and memories of the resulting 200,000 deaths remain fresh.

Military officials in Venezuela and, especially Colombia spend much time denying rumors that they are about to march forth. What Americans would regard as dangerous instability there appears more and more to be simply the natural state of this peculiarly Latin American style of democratic government.