Fidel Castro learned a lot from Chilean President Salvador Allende's failed power grab in 1973. And he used the lessons of that bitter defeat to coach Venezuela's Hugo Chávez to dictatorship under the guise of democracy more than 25 years later.

Now Latin America's revolutionaries may be experiencing another setback and this time they can't claim that a military coup removed their would-be dictator. Instead, former Honduran president Manuel Zelaya was arrested by order of the Supreme Court and deposed by Congress. And despite enormous international pressure, the Honduran democracy has so far defended its rule of law.

Yet far from giving up, Castro protégés are already using what they learned in Tegucigalpa in El Salvador. Central America's most promising free-market democracy is now fighting for its life.

Allende got the boot from his military because he had been trampling the constitution. The Supreme Court, the Bar Association and the Medical Association all denounced his disregard for the rule of law. According to James R. Whelan, author of a history of Chile titled "Out of the Ashes," the lower house of its Congress passed a resolution on Aug. 22, 1973, that "said bluntly that it was the responsibility of the military . . . 'to put an immediate end' to lawlessness and 'channel government action along legal paths . . . .'" Less than a month later, the military complied.

The lesson from Chile for the hard left was that success depended on first getting control of the institutions with the power to check an aspiring tyrant. Now the leadership of El Salvador's FMLN party, composed of many former guerrillas, is attempting just that.

It took some 20 years for the political party of the FMLN to get to the presidency. Many Salvadorans distrust it because of its violent history. But FMLN candidate Mauricio Funes ran as a moderate. The economy had suffered under former President Tony Saca of the center-right Arena Party. Disillusioned Salvadorans sought change.

Mr. Funes is still widely viewed as a moderate. Last week one former president, Alfredo Cristiani, told me in a telephone interview that he believes Mr. Funes is "genuinely not part of the group inside the FMLN that wants to take El Salvador to a dictatorship."

Yet Mr. Cristiani is worried, and with good reason. There are plenty of extremists around Mr. Funes, starting with José Luis Merino, who is commonly believed to be the party's de facto leader. His nom de guerre, "Ramiro," showed up as an ally in correspondence among leaders of the Colombian guerrilla group FARC that were captured by the Colombian military in 2008.

A couple years back Mr. Merino explained in a media interview the FMLN's political agenda this way: "It is to take power, to conquer the entire nation and, in that way, assure that the form of government does not change. Of course, not with bayonets or persecution. There are examples, like Venezuela, that is our model."

The institutions that stand in Mr. Merino's way are the congress, the Supreme Court and the electoral council. The party tried to wrest control of the high court's constitutional panel, in collaboration with Mr. Saca while he was still president. Luckily, the backroom deal was challenged and the rule of law prevailed.

But the event showed that the FMLN really is following Mr. Merino's "Venezuela model." It also suggested that, just as critics have warned, Mr. Saca may be willing to help the FMLN. The former president knows that it is not uncommon for an incoming political party to investigate a former president. If Mr. Saca has anything to hide, the best chance of doing so would be to make sure there is no investigation.

Speculation about such political machinations increased last month when 12 Arena congressmen announced a break from their party. Calling themselves "independents," they proceeded to vote with the FMLN against an investigation Arena wanted into abuses of agricultural subsidies.

What prompted the defection? Mr. Cristiani told me that a high-ranking member of the Christian Democratic Party (PDC) has told him that at least one PDC congressman has been offered $700,000 to vote with the FMLN. Separately, the secretary general of the PDC, Rodolfo Parker, has publicly warned of multiple offers from a middleman of between $300,000 and $500,000.

Mr. Saca denies any involvement in the vote-buying scheme, and surely Mr. Merino has enough motivations to act on his own. But rumors are swirling in the Salvadoran press about links between individuals close to Mr. Saca and alleged middlemen acting on behalf of Mr. Merino.

The Arena defection is no ordinary betrayal of the electorate. In Salvador voters choose a party ticket. Congressmen are named according to how many votes the party gets. These congressmen were not elected as individuals but rather as representatives of the elected party. With their votes the FMLN is now only one or two votes short of a two-thirds majority. If it gets that majority, the party can tell the moderate Mr. Funes what to do. Then Chávez acolytes will be well on their way to winning what their bedfellows could not in Honduras.