A little more than 64% of the electorate voted against him in the five-person race. His victory margin was the smallest in Mexican history. The campaign aggravated divisions between rich and poor, north and south, over rival formulas for creating jobs and fighting poverty. Many voters doubt he won Sunday's election fairly. And his conservative National Action Party, or PAN, ended up with a minority in Congress.
As the last official returns trickled in Thursday, giving him a half a percentage point edge over leftist Andres Manuel Lopez Obrador, Calderon set aside the acrimonious language of the campaign and appealed for reconciliation.
He said, "The message the voters have given us is clear: 'Work together, join your priorities, put politics aside, and agree for the good of Mexico.' "
But the 43-year-old Calderon's ability to heal and govern the country will be severely tested.
Not since the early years of the Mexican Revolution, before the Institutional Revolutionary Party, or PRI, consolidated its rule in the late 1920s, has a national leader faced the prospect of taking office with such limited popular legitimacy and political power.
PRI presidents handpicked their successors and ratified them in rigged elections. Carlos Salinas de Gortari was the first PRI candidate to face serious voter opposition, but his tainted victory in 1988 did not diminish his ability to govern; his party controlled Congress.
PAN's Vicente Fox, who broke the PRI's monopoly on the presidency in 2000, took office with minority support in Congress but with a mandate for change and a status as the country's first truly democratic leader.
If Mexico's electoral tribunal overrules Lopez Obrador's legal challenge, Calderon will assume office Dec. 1 with neither Salinas' power nor Fox's charisma.
"He should be happy he won, but rather than celebrate too much I hope he understands he has a huge challenge," said Juan E. Pardinas of the Center of Research for Development, a Mexico City think tank. "And the biggest challenge may be to understand the Lopez Obrador voter. He'll have six years to learn why Lopez Obrador was so popular to understand the frustration within the Mexican population over the current state of the economy."
Calderon, an economist and lawyer, promised to be the "jobs president." He insisted that relief for Mexico's 50 million poor required a bigger dose of the Fox administration's open-market policies. Lopez Obrador turned plaza rallies into indictments of the privileged.
In campaign speeches and TV spots, Calderon ridiculed the idea that the government could afford national welfare programs or massive public works, proposals that drew millions of Mexicans to his opponent. He played on the fears of middle-class voters, implying that under Lopez Obrador they could lose what little they had gained during Fox's presidency.
Mexico's prosperous north voted overwhelmingly for Calderon and his business-friendly PAN, whereas Mexico City and the poorer southern states with large indigenous populations backed Lopez Obrador and his Democratic Revolution Party, or PRD.
Forging a consensus among the political and social groups may be tougher than winning the election.
"There's a lot of bitterness and bad blood between the two campaigns, and this is the perfect formula for gridlock," said Armand Peschard-Sverdrup, a Mexico scholar with the Center for Strategic and International Studies in Washington.
Calderon's proposals for expanding the Mexican economy are complicated. Most need congressional approval. PAN lawmakers will be the largest bloc in Congress, but they will make up only about one-third of Mexico's Senate and lower house; the PRI and PRD will each hold less than 30%.
Getting laws passed will require consensus building — a goal that eluded Fox, whose proposals were shot down by the PRI. Calderon does have some experience with Congress. He served as Fox's congressional leader in the lower house and twice shepherded the president's federal budget to passage.
For weeks, Calderon has promised to create a mixed-party Cabinet — an offer he repeated Thursday. His advisors said formal talks with opposition parties were still days away, although Calderon this week publicly offered a Cabinet post to Lopez Obrador.
Arturo Sarukhan, a leading Calderon advisor, said Fox failed to win approval for his ambitious economic proposals because he brought sympathizers of opposition parties into his Cabinet without getting their commitment to rally support in Congress.
"With Calderon there will be a quid pro quo between a place in the Cabinet and the delivery of votes for his reforms," he said.
Otherwise, Calderon would face a formidable opposition if lawmakers from the PRI, whose presidential candidate, Roberto Madrazo, finished a distant third, team up with the PRD to block his proposals.
But Sarukhan said there were "strong undercurrents within the PRI that have been shaken by its showing in the polls and that are convinced of the need to play a new game in Mexican politics — the creation of consensus."
Political analysts said creating a unity Cabinet was Calderon's most urgent task.
"A Cabinet that is plural, with the best people, irrespective of party affiliations, will help with the message that 'I have six years to govern and I need your help to do it,' " said Andres Rozental, a former Mexican diplomat and presidential advisor.
Assembling such a Cabinet "is going to take a lot of effort on his part, but he's willing to make that effort," Rozental said. "He understands he cannot govern on his own."
Complicating that task is a conviction among many of the 14.8 million who voted for Lopez Obrador that the election was stolen from their candidate.
Commentator Sergio Aguayo, who has long campaigned for free elections, said he had little doubt that the electoral tribunal would uphold Calderon's victory.
"But the question of legitimacy is more ethereal because it does not depend on any tribunal," he said. "You are legitimate because the people give you legitimacy. And this will be difficult for Calderon to achieve because the ideological conflict in this country is so intense."
Calderon would move into Los Pinos, the presidential residence, surrounded by political adversaries. The newly elected mayor of Mexico City used to work for Lopez Obrador, and the state of Mexico, which encircles the capital, has a PRI governor.
The new president could best unite the country by showing that he can make good on his promise to create jobs, said Homero Aridjis, a poet and social commentator.
"He's taking over an extremely divided Mexico," Aridjis said, "polarized by decades of lagging social programs and by presidential candidates who used the divisions between rich and poor as part of their campaign rhetoric. He has an almost impossible job."