According to political scientist Franco Martínez, the Patriotic Party is likely to capture approximately 40% of the 158 congressional seats and the National Unity of Hope (UNE) and Grand National Alliance (Gana) coalition is likely to capture another 25%. Not having a presidential candidate will obviously hurt UNE-GANA's congressional performance.
Manuel Baldizon's LIDER, Eduardo Suger's CREO, and the "narco" Union of National Change (UCN) fill out the top five. These three parties and the remaining smaller political parties such as the EG, PAN, PU, and Frente Amplio, will share approximately 35% of the seats.
The Guatemalan congress' website is down so I don't if the number have changed recently, but here's what the distribution of congressional seats have looked like over Colom's entire term in office.
UNE-GANA will likely lose some seats after September 11th compared to what they currently have. There's also a good chance that they will lose some of those seats that they do win next month when those elected defect to other parties or become independent.
LIDER didn't exist at the beginning of the last congress and according to a recent Wikileaks on Plaza Publica's website, it helped amass 28 congressional seats by offering $61,000 to any member of congress that switched to its bench. And, no, that's not illegal. Given Baldizon's rising support, the party should do well in congress, but perhaps not quite as well as it is doing right now.
Again, be careful to read too much into these numbers. It's much more difficult to extrapolate from these surveys to the composition of congress than it is to the likely outcome of the presidential contest.