MEXICO CITY — After 18 years of establishment politics, Mexicans decided on Sunday that enough was enough, electing the leftist candidate Andrés Manuel López Obrador as president in a landslide victory in an election of firsts.

Capturing more than half the vote, according to early returns, he won by the largest margin in a presidential race since the nation transitioned to democracy nearly 20 years ago.

Here are five takeaways from the stunning victory of Mr. López Obrador and what to expect:

Rejecting the status quo

To grasp the stark repudiation of the establishment, and Mr. López Obrador’s ability to capitalize on it, it is helpful to understand the recent electoral history of Mexico.

President Enrique Peña Nieto’s Institutional Revolutionary Party, known as the P.R.I., dominated politics in Mexico from 1929 until 2000, when it was unseated by the conservative National Action Party, or P.A.N., at the time led by Vicente Fox.