By Mary Anastasia O'Grady

Bogotá

Colombians go to the polls on Sunday to choose a successor to two-term President Álvaro Uribe. Nine candidates will be listed on the ballot, but the race is really about only two of them.

In one corner is former Uribe Defense Minister Juan Manuel Santos of the "U" Party. His challenger is Antanas Mockus, of the Green Party, a former dean of the National University of Bogotá and a two-time mayor of Bogotá. Neither is expected to garner the 50% plus one of the votes required to win in the first round. A potential run-off is too tight to call.

In a country where the violent criminal network of the Revolutionary Armed Forces of Colombia (FARC) has spent more than 50 years murdering, maiming and kidnapping civilians, Mr. Santos would seem to be a shoo-in. Besides being a former trade minister and a former finance minister in two other Colombian governments, he headed the ministry of defense from July 2006 to May 2009. During his tenure Colombia pulled off a string of brilliant victories against the FARC.

Mr. Santos was at the helm when Colombia raided a FARC camp in Ecuador in 2008, killing comandante Raúl Reyes and capturing volumes of internal rebel documents that provided crucial intelligence. He was also in that post during the dramatic July 2008 rescue of former presidential candidate Ingrid Betancourt and 14 others, including three Americans.

This success may be working against Mr. Santos. Many urban voters, especially the young, seem to believe that with the rebels severely weakened security is no longer the issue of national survival that it was eight years ago. These Colombians have turned their attention to Mr. Mockus and his promises to give them a government that is fairer and more honest.

Mr. Mockus has positioned himself as the candidate of change. Until March he wasn't even polling at 10%. But an emerging corruption scandal involving a government agency known as the Department of Administrative Security (DAS), which answers to the president, has energized his campaign.

The agency is accused of wiretapping and following a variety of public figures it believed could have links to drug traffickers and rebels. These activities are not expressly forbidden under Colombian law, but wiretapping requires permission from a judicial official. Whether that permission was received is still not clear. But since those subject to surveillance allegedly included members of the Supreme Court and since the court has had a number of high-profile disputes with Mr. Uribe, the conventional wisdom, crafted by the local press, is that the government was using the intelligence agency as a political weapon.

Mr. Uribe has denied any knowledge of illegal activities at the DAS and called for it to be closed. Mr. Santos had no connection to the DAS and even fired two top national police officers in 2006 due to illegal wiretapping, but he has been painted as part of an establishment that allowed this to happen.

Ironically, the DAS allegations highlight how real the narco-trafficking threat to democracy remains. What happened at the agency won't be easy to unpack for the same reason that it is nearly impossible to prosecute narco-trafficking while respecting civil liberties. As the late economist Milton Friedman once explained to me when I asked him about the costs of the drug war, wherever two parties are engaged in a voluntary transaction it is impossible to police that transaction without informants. And once you have informants, Friedman pointed out, you have fertile ground for corruption.

Thus it would hardly be a surprise to learn that elements inside the DAS strayed from the rule of law. But it is also obvious that organized crime has a strong incentive to inflict damage on the government. Given that, in practice, there is no punishment for lying under oath in this country, the eagerness of a judiciary, known to have major political differences with Mr. Uribe, to give credence to testimony from shady witnesses also muddies the water. On Friday the Colombian daily El Tiempo reported that the signature of Mr. Uribe's lawyer had been falsified in an effort to link him to DAS corruption.

The moral of the story is that Colombia needs not only economic policy reform, such as simplifying the convoluted tax code, but also improvements in the judiciary and intelligence-gathering that would better protect its institutions from organized-crime infiltration.

Even Mr. Mockus's critics acknowledge his appeal, stemming from the heavy emphasis he has put on upholding the rule of law in a country plagued by corruption. But he lacks national experience, tends to address problems like an academic, and heads a party that has only two seats in Congress. The choice voters face is between taking a chance on Mr. Mockus, an unknown quantity, and going with the more proven product they see in Mr. Santos. After all, as Mr. Santos told me in an interview here on Friday, when it comes to securing the peace "we are winning, but we have not won yet."