Peru holds the second round of its presidential election on Sunday. The runoff pits economist Pedro Pablo Kuczynski against Keiko Fujimori, the daughter of former president Alberto Fujimori (1990-2000), who is serving prison terms on corruption and human rights abuse charges.

Yet even before Sunday’s votes are cast, we already know one key electoral result: Fujimori’s party, Fuerza Popular (Popular Force), will hold a solid majority — 73 of 130 seats — in Peru’s unicameral congress.

Congressional elections were held concurrently with the first round of presidential balloting on April 10. Despite Peru’s proportional electoral rules, which we will discuss below, Popular Force won 56 percent of the seats with just 36 percent of the vote. It will hold Peru’s first congressional majority since Alberto Fujimori’s presidency in the 1990s. The outcome surprised nearly everyone in Peru, including the Fujimoristas.

How did this happen? And what are the implications for the runoff, and for how Peru will be governed in the upcoming term?

Peru’s rules for electing congress

Peruvian congressional elections are by list proportional representation. Voters indicate preferences for slates of candidates nominated by parties in each district. The congress’s 130 seats are awarded across 26 districts that elect anywhere from one seat, in Madre de Dios, to 36 seats, in Lima. Most districts elect between two and five representatives. Parties must win 5 percent of the nationwide vote to be eligible for seats, and the formula used to convert votes to seats is the D’Hondt method — which is important, as we’ll see below.

After April’s first-round vote, many observers attributed Popular Force’s surprising legislative majority to that 5 percent threshold, which blocked some of its smaller challengers from winning seats. As we will explain below, the threshold mattered to the outcome, but not nearly so much as the formula for converting votes to seats.

Using the district-level votes from the National Office of Electoral Processes (ONPE), we reproduced the election results under the existing system and also simulated results that could have been obtained under a variety of proportional representation formulas used around the world, both with and without the legal threshold.

Peru’s threshold denied representation to two parties that, based on the votes cast in April, would have won seats. The leftist Democracia Directa, which won 4.3 percent of the vote, would have captured five seats, and former president Alejandro Toledo’s Peru Posible would have earned one. Instead, most of those seats went to Keiko Fujimori’s Popular Force, which gained four seats more than it would have earned with no threshold. Yet even without those seats, Popular Force would hold a majority of 53 percent.

But what’s more important is the formula used to award seats

Far more consequential was Peru’s use of the D’Hondt formula. Some technical background: The most commonly used proportional representation formulas are D’Hondt and the Hare Quota with Largest Remainders (HQLR) Under D’Hondt, each party’s vote total is divided by a sequence of increasing integers, and seats are awarded according to the largest quotients until all the district’s seats are distributed. Under HQLR, parties effectively spend their votes to purchase seats at a price (or quota) determined in each district by the number of votes cast and seats available. Once no more parties have full quotas to spend, the remaining seats are distributed at “discount prices” to parties that fell short of the quota.

Electoral systems scholars have long noted that the D’Hondt method tends to confer big seat bonuses on large parties, whereas HQLR is relatively friendlier to smaller lists. In Peru’s recent vote, the differences in outcomes between the systems are particularly stark. Under HQLR, Popular Force’s 36 percent vote share would have translated into 41 percent of seats, even with the legal threshold shutting out smaller parties — and 40 percent without it.

Had Peru used any of the other PR formulas employed in other democracies around the world (the Droop quota, the Sainte-Laguë divisor or modifications thereof), with or without the legal threshold, Popular Force would not have won a congressional majority in April. Instead, Peru would be looking at a party configuration much like what it has faced in the three terms since Alberto Fujimori fled the country in disgrace. There would be no majority party, and the president would have to piece together a coalition to legislate.

Fujimori’s congressional majority may profoundly affect Peru’s future

Fujimorismo’s unexpected legislative majority will have important consequences for Peruvian democracy. Peru’s congress is hardly one of Latin America’s most effective. Its oversight capacities are limited and, due to weak parties and low reelection rates, Peruvian legislators are inexperienced (more than 70 percent are first-termers), undisciplined, and operate with short time horizons.

Nevertheless, Peruvian presidents have been surprisingly constrained in the 15 years since Alberto Fujimori governed. Lacking legislative majorities, presidents Alejandro Toledo (2001-2006), Alan Garcia (2006-2011) and Ollanta Humala (2011-present) were forced to build multiparty coalitions. When these coalitions collapsed, as Humala’s did in 2015-2016, they lost control of the congress and were weakened.

In the past decade, the Peruvian congress has started to oversee and at times check executive power. Under Garcia, for example, the congress carried out a high-profile investigation into the 2009 violence between police and indigenous protesters in Bagua, which left 33 people dead. In June 2009, the congress held a censure vote that, while unsuccessful, contributed to Prime Minister Yehude Simon’s resignation a week later. Under Humala, the congress launched an investigation into domestic espionage by Peru’s intelligence agency. It then successfully voted to censure Prime Minister Ana Jara — the first censure of a Peruvian prime minister since 1963 — forcing her to resign.

Why should we be concerned about Fujimorismo?

Whoever wins Sunday’s runoff, Peru’s executive and legislative branches will have a very different relationship. Kuczynski, a political liberal whose allies hold only 18 of 130 seats, would have to negotiate an arrangement with Popular Force – a situation that could make Peru very difficult to govern. Although Fujimorismo’s economic program differs little from Kuczynski’s, its cooperation may require concessions in other areas — such as the pardon (or move to house arrest) of the disgraced Alberto Fujimori. And if the 77-year-old Kuczynski were to lose public support, as has each of his three predecessors, the Fujimorista majority could be tempted to remove him early.

Under Keiko Fujimori, the primary danger would be the concentration and abuse of presidential power. A Fujimori presidency would be largely unchecked by Congress or the judiciary. Fujimorismo’s legislative bloc has always been disciplined. Control of the executive branch — with its patronage and other resources — would initially fortify that discipline, allowing Keiko Fujimori to govern unilaterally. She would have little difficulty, moreover, filling Peru’s judiciary with allies.

Majoritarian government would not necessarily be problematic if Peru’s democratic institutions were strong and Fujimorismo were committed to liberal democracy. But neither is true. Although Keiko Fujimori has publicly distanced herself from her father’s authoritarianism — declaring, for example, that she would not have closed Congress in 1992, as he did — neither she nor other leading Fujimoristas have broken with Alberto or even acknowledged that he committed crimes. Most Peruvians (both pro- and anti-Fujimoristas) believe that Keiko Fujimori will pardon her father. Many believe that he would then exert considerable influence behind the throne.

Keiko Fujimori has purged much of the Fujimorista old guard from Popular Force. Still, the new Fujimoristas inspire little confidence. The party’s general secretary and principal financier, Congressman Joaquin Ramirez, is reportedly being investigated by the U.S. Drug Enforcement Agency for drug trafficking and money laundering. And last week, in a bizarre move reminiscent of some of the shadier actions by Alberto Fujimori’s shadowy adviser, Vladimiro Montesinos, Keiko Fujimori’s vice-presidential candidate, José Chlimper, sought to discredit the charges against Ramirez by orchestrating the broadcast of a doctored audio tape on a television news program. Popular Force leaders showed their intolerant tendencies during the campaign, attacking anti-Keiko Fujimori demonstrators as “terrorists.”

Observers of Peruvian politics differ over how much (or how little) Fujimorismo has changed since Alberto’s fall from power. Some believe the party remains unreconstructed – and controlled by Alberto. Others believe the party has moderated somewhat under Keiko Fujimori. But few, if any, observers believe that the Fujimoristas are committed liberal democrats.

The stakes in Sunday’s presidential runoff are high, but April’s legislative outcome has already reshaped Peru’s political terrain. With its unexpected legislative majority, Fujimorismo — a party associated with corruption and criminal activity – will be able to shield itself from legislative and judicial oversight. And Peru’s still-fragile democracy could face the twin threats of a disciplined presidential majority and a governing party with illiberal — and perhaps authoritarian — tendencies.

John Carey is the Wentworth professor in the social sciences at Dartmouth College.

Steven Levitsky is the David Rockefeller professor of Latin American studies and professor of government at Harvard University.