The party of Mexico's president-elect Andrés Manuel López Obrador and its allies will have an outright majority in Congress but will not surpass the two-thirds threshold needed to change the constitution, estimates show.
Mr. López Obrador, a leftist, won a landslide victory on Sunday and will wield considerable power in a Congress set to have the highest concentration of female lawmakers in Mexican history.
For the first time, there may be more female senators than male, and a nearly 50-50 split in the lower house, Mexico's electoral institute projects, underlining the far-reaching consequences of López Obrador's win.
Tapping into widespread disillusionment with years of violence, corruption, and inequality in Latin America's No. 2 economy, López Obrador won about 53 percent of votes.
His victory has upended Mexico's political status quo, in which just two parties have held the presidency since 1929, and drawn a line under decades of technocratic rule. No ruling party has held an absolute majority in Congress since 1997.
López Obrador's triumph extended to both houses of Congress, where his National Regeneration Movement (MORENA) party and its partners are seen winning 61 percent of seats in the 500-member lower house, projections by pollster Consulta Mitofsky show.
His coalition, which includes the leftist Labor Party and the Social Encounter Party (PES), a religious, socially conservative anti-abortion group, is projected to win 53 percent of seats in the 128-member Senate, Mitofsky said.
The congressional advantage will help López Obrador deliver on his pledge to radically reshape Mexico, which he intends to do by helping the poor while also keeping investors happy.
"It makes the day-to-day easier," Carlos Urzúa, López Obrador's pick for finance minister, told Reuters on Monday.
Though the estimates suggest López Obrador will lack the two-thirds majority in both houses needed to make constitutional changes, he may still benefit from defections in Congress. Mr. Urzúa said the government had no plans to change the constitution.
Final congressional results are expected by Saturday.
The results are a severe blow to the established parties.
The ruling Institutional Revolutionary Party (PRI) which governed Mexico continuously from 1929 to 2000, and again from 2012, is seen with 45 seats in the lower house, according to Mitofsky, down from over 200 in the 2012 election.
The PRI is likely to get 13 seats in the Senate, Mitofsky said, down from over 50 six years ago.
Tainted by corruption scandals, the PRI has become increasingly reviled under current-President Enrique Peña Nieto for failing to get a grip on security and lift the economy.
The National Action Party, which ruled between 2000 and 2012 and became embroiled in a grisly drug war, was seen winning 82 seats in the lower house and 24 in the Senate, according to Mitofsky, well below their 2012 tallies.
Still, López Obrador's coalition may prove unwieldy. He will have to balance interests of leftist nationalists, social liberals, and religious conservatives.
So far López Obrador has kept a lid on the potential conflict by adopting ambiguous stances on contentious issues such as abortion, gay marriage, and economic liberalization.
But his tie-up with the PES may prove tough, and it faces resistance from supporters on the left.
This story was reported by the Thomson Reuters Foundation.