The past two non-Peronist leaders since democracy was restored in 1983 failed to complete their terms.

Both were from the Radical Party. Raul Alfonsin stepped down six months early in 1989 as hyper-inflation raged, and Fernando de la Rua fled the presidential palace in a helicopter during the 2001-02 depression.

Lacking a majority in Congress, Macri is likely to move quickly with his reform agenda during an expected early honeymoon period and try to draw on cross-party support.

He is expected to court the backing of lawmakers loyal to centrist Sergio Massa, who placed third in the first round of the presidential election last month, and other smaller parties to outmaneuver Fernandez hardliners.

Control over discretionary transfers that boost provincial budgets, meanwhile, will give Macri leverage over governors and in turn their senators in the upper house.

His "Let's Change" alliance will control Argentina's three main bases of power: the federal government, the populous Buenos Aires province that traditionally was a Peronist stronghold, and the capital city.

"This is a huge shift for Argentine politics," said Juan Cruz Diaz, head of the Cefeidas Group.

"For years the opposition, controlled by an unsuccessful Radical Party, claimed it was impossible to govern with Peronism in opposition. Macri has a big opportunity to prove that wrong".