PORT-AU-PRINCE, Haiti — After spending more than $33 million on a widely discredited election in Haiti, the United States has been pressing the country’s leaders to go ahead with a presidential runoff election this Sunday, despite a growing chorus of warnings that the vote could lead to an explosion of violence.

Haitian leaders, political parties and others have denounced the first round of voting in October as a fraud-riddled fiasco and protested in the streets to stop the runoff. One of the two remaining candidates says he is boycotting, effectively making it a one-person race.

President Michel J. Martelly took to the airwaves on Thursday to warn that protests on Election Day would not be tolerated. Civic, business and religious leaders are engaged in tense back-room negotiations to broker a deal in an effort to avoid violence and put off the race. Eight election observer organizations have pulled out over the fraud accusations and chaos, including a Haitian group funded by the United States.

But the Obama administration — which spent the money to help ensure “credible, inclusive, and legitimate” elections here — has emerged as a central force, pressing Haitian politicians to move forward with the vote, however flawed the first round may have been.