Last Friday it was a major drug lord in Ciudad Juárez. Over the weekend came the capture of the alleged head of meth production of a group terrorizing the state of Michoacan. And on Tuesday Mexican authorities said they caught a major figure of a splinter group in Acapulco.

But each piece of positive news the government showcases seems inevitably to come paired with a setback or other negative development in a battle against organized crime that has already seen some 40,000 killed since Mexican President Felipe Calderon took office in late 2006.

The most recent batch of arrests were heralded at a press conference earlier this week, during which Mexico’s national security spokesman Alejandro Poire listed off names of captured suspects and touted the government’s success in battling crime. “Important criminal operators have fallen one after the other, with the growing capacity of federal forces,” he said.

But as the arrests were announced, pollsters in the state of Michoacan went missing. They have since been freed, and details of what actually happened are murky. But most believe one of the two major crime groups, La Familia and the Knights Templar, were behind their abductions. Now many worry that organized crime could hamper efforts to gauge public opinion and hence influence upcoming state elections scheduled for November.

Targeting pollsters would be a new intimidation tactic, but would not be surprising. In previous elections, candidates have been targets: the highest profile case was the assassination last year of Rodolfo Torre, who was running for governor of Tamaulipas. Sitting candidates, especially at the mayoral level, have also been increasingly counted among the death toll in Mexico.

And even an event considered a boost for the ruling National Action Party (PAN), which is struggling to hold onto power as state and presidential elections approach, may have a downside as well.

The capture last week of José Antonio Acosta Hernández, suspected of leading the drug-trafficking gang La Línea, which works with the Juárez Cartel, was hailed by the government. Mr. Poire said the arrest "is without a doubt a blow against this criminal structure without parallel.” But it came with the accusation of his being behind 1,500 killings – a sign if any of how badly the city has deteriorated in the past five years.

Mexico arrested a suspected drug trafficker in Acapulco allegedly behind the kidnapping of 20 Mexican tourists last September, many who were later found in a mass grave. But will he or any of these suspects ultimately face trial?

As major arrests were happening, the government announced Monday that 21 of 31 top state and federal district prosecutors had resigned en masse.

No concrete explanation was given for their sudden departures. But a day after the announcement, President Calderon tweeted a “congratulations” to new Attorney General Marisela Morales for the ongoing purge in her office.

Her efforts might prove how serious Calderon is about solidifying institutions, which will make for a strong platform ahead of the 2012 race. But the levels of corruption she is contending with might ultimately mean that today’s good news might be tomorrow's embarrassment. The former mayor of Cancun was recently let go after a case built on drug trafficking and money laundering charges fell apart, and efforts to try the former mayor of Tijuana on weapons charges also failed this summer.