Where could those voters go?
Zuluaga's supporters seem likely to stick with their man in the second round. After all, someone who voted for a relatively unknown, extremist candidate probably did so for a reason. Even the hacker scandal doesn't appear to threaten his support.
Santos was undoubtedly the default choice for many, and so his voters could presumably defect if given a strong reason. On the other hand, many voters whose first choice was eliminated will prefer a known quantity to a leap into the relative unknown with a man like Zuluaga.
Martha Lucia Ramirez 15.5%
The Conservative Party's candidate is seen as closer to Zuluaga, and on election night even appeared with ex-Pres. Pastrana, who made a speech criticizing Pres. Santos. But the Conservative Party has been divided, with many of its representatives backing Santos. Ramirez's relatively strong showing might pull more supporters away from Santos. Ramirez and Zuluaga have both harshly criticized the peace negotiations going on in Havana, Cuba, perhaps making them natural allies. On the other hand, Ramirez made anti-corruption a major plank in her platform - potentially making it difficult for her to ally with the scandal-embroiled Zuluaga.
And many of Ramirez's voters likely supported her primarily out of loyalty to the Partido Conservador, one of Colombia's two traditional establishment political parties (along with the Liberales). With the Conservador label missing in the second round, will a lot of those voters stay home? And, these traditional, establishment Colombians may feel more comfortable voting for Santos, a member of one of Colombia's most important families, than a relative unknown like Zuluaga.
Santos's best strategy to capture Conservative Party voters could be to try to frame Zuluaga as a corrupt, right-wing nut - which may not be very unrealistic.
If Zuluaga can capture Ramirez's voters and hold onto his own, he'll have victory virtually sewn up.
Clara Lopez 15.2%If you want to bet on one thing, bet that Lopez's supporters will never vote for the right-wing Zuluaga.
Lopez represents the union of the Polo Democratico Party, which is often seen as sympathetic to the ideology of Colombia's leftist guerrillas, and the reborn Union Patriotica Party, which was linked to the FARC guerrillas in its birth during the 1980s, but was destroyed when thousands of its leaders were assassinated by right-wing groups, probably linked to the government.
While Lopez voters won't support Zuluaga, their far-left ideology and distaste for the establishment may prevent them from backing Santos, either.
With these voters, Santos gets a big boost from the ongoing peace negotiations with the FARC guerrillas. The FARC clearly want Santos reelected, which is why they signed an agreement on ending the drug trade just a week before the first-round vote.
Will the left swallow hard and support Santos, who's generally seen as a center-right politician and who led the war against the guerrillas as Pres. Alvaro Uribe's defense minister? One sign that they could was the departure of several top staffers of Bogotá Mayor Gustavo Petro - himself a one-time M19 guerrilla leader - to work on Santos's campaign.
Will Lopez herself campaign for Santos? Perhaps - if only to stop Zuluaga.
Enrique Peñalosa 8.3%
After almost pulling even with Santos in some polls early in the campaign, Green Party candidate Peñalosa, en ex-mayor of Bogotá, really tanked on election day. Nevertheless, Peñalosa's voters will be crucial on election day because they're the most up-for-grabs.
Despite its name, the Colombian Green Party, an artificial creation, has a confused ideology (if it has any ideology at all). Peñalosa has in the past allied himself with ex-Pres. Uribe, an arch-conservative. During this campaign, he allied his party with leftist Bogotá Mayor Gustavo Petro.
Despite this ideological confusion, Peñalosa was certainly closer to Santos than Zuluaga, particularly Peñalosa's support for the government-guerrilla peace talks in Havana.
Santos needs to court the often-difficult Peñalosa (perhaps by offering him a position as minister?), to have a chance of getting over the top.
To cobble together the 50% of voters he needs to win, Santos must hold onto his own 25%, capture a substantial portion of Conservative voters, get most of the Polo/UP's supporters and get Peñalosa's people behind him. But even all of that might not add up to 50%.
The Abstainers 60%
A record 6 out of 10 eligible voters stayed home on election day. Those millions of potential voters probably represent more of a potential for Santos than Zuluaga, since conservatives and supporters of more extreme candidates tend to be more dedicated voters. Therefore, a much higher proportion of Zuluaga supporters than Santos supporters likely voted on Sunday. Can Santos get the barrios and university students to turn out for him?