The party of Mexico's President Vicente Fox has suffered defeat in key mid-term legislative elections, according to early partial results.
Voters are re-electing the whole lower house of Congress
All 500 seats in the lower house - the Chamber of Deputies - are being contested, as are governorships in six of the country's 31 states, including Mr Fox's home state of Guanajuato.
Returns from about half of the country's polling booths showed that Mr Fox's National Action party (PAN) won 31% of the vote and would lose at least 40 seats of the 202 it currently has, said Jose Woldenberg, president of the Federal Electoral Institute.
Mr Woldenberg said that PAN's main rival - the Institutional Revolutionary Party (PRI) - took 34.4%, and would have between 222 and 227 seats - up from 207.
Today it has been confirmed that we are the leading political force
Roberto Madrazo, PRI leader
The partial results suggested that the biggest winner appeared to be the leftist Party of the Democratic Revolution (PRD) which looked like increasing its number of seats from 56 to 99.
In the gubernatorial race, PAN conceded its the loss of the key border state of Nuevo Leon and was trailing in two other states.
However, the party appeared to be leading in two central states of San Luis Potosi and Queretaro.
"Today it has been confirmed that we are the leading political force," PRI leader Roberto Madrazo was quoted as saying by the Associated Press news agency.
The elections are seen as the first real test for President Fox since he came to power in 2000, ending more than 70 years of PRI's one-party rule.
The BBC's Nick Miles in Mexico City says it will now be even harder for Mr Fox to push through economic reforms, like opening up the oil industry to private investment.
Our correspondent says that unless Mr Fox now can build coalitions, his remaining three years in office will be spent in a legislative strait-jacket.
'Presidency of stagnation'
When Mr Fox won the presidency he promised Mexicans change - a vibrant economy and a move away from what many people had grown to see as the corrupt politics of the PRI.
Fox: Elections could be prelude to stagnation
Three years on and President Fox is not up for re-election - it is the deputies of his party, the PAN, that are.
Whilst Mr Fox's approval ratings remain high, there is a degree of disillusionment amongst the electorate that his government hasn't brought the promised changes.
Changes have come - there is new anti-corruption legislation and transparency laws.
But the economy has suffered because of the economic downturn in the United States and the PAN has suffered as a result.
Correspondent say it now looks increasingly likely that Mr Fox' last three years in office will be like a presidency of stagnation.