Outgoing President Felipe Calderon has sent a bill to Congress that would establish a two-round system of voting in future Mexican presidential elections.

He is proposing a constitutional change requiring a runoff election when no presidential candidate garners more than 50 percent of the ballots in the first round.

The runoff would pit the two candidates who received the most votes in the first go-around.

The bill, sent Tuesday to the lower house of Congress, proposes that the first round of presidential voting take place on the first Sunday in July and the second round - if necessary - be held on the second Sunday in August.

Calderon, of the conservative National Action Party, or PAN, won just 35.89 percent of the vote when he was elected for a six-year term in 2006, while runner-up Andres Manuel Lopez Obrador received 35.31 percent of the ballots.

Roberto Madrazo, of the Institutional Revolutionary Party, or PRI, the authoritarian party that had governed uninterruptedly for 71 years until 2000, finished a distant third in that year's election.

Lopez Obrador and his leftist PRD party denounced Calderon as a "spurious president" whose ostensible victory was the result of machinations by big business and the administration of outgoing President Vicente Fox, also of National Action.

Calderon, who will hand over power to the PRI's Enrique Peña Nieto on Dec. 1, last week proposed changing the country's official name from United Mexican States to Mexico. EFE