As a researcher who has interviewed him half a dozen times over the past decade, I can attest to his striking capacity, born of a career in intelligence, to say what his listener most wants to hear. It is easy to leave an initial interview convinced that he is the peacemaker and democratizer Guatemala needs. But as you measure his words against his deeds over time, you figure out that Mr. Pérez Molina is a master of manipulation.

He is playing the United States now. It is nonsensical to assume that a president who has tolerated, if not reaped the benefits of, a thoroughly corrupt administration is suddenly willing to purge it.

Furthermore, it requires a tremendous leap of imagination to accept that the president has no ties to the scheme involving the customs fraud. That network initially emerged during Guatemala’s 36-year civil war, and its key players include several members of the military with close ties to Mr. Pérez Molina.

American support for Mr. Pérez Molina has bought him time to consolidate an unholy alliance with the equally, if not more corrupt, major opposition party, which has set its sights on winning the coming elections. The criminal networks that control politics also seem to have reaffirmed their influence over the judicial system: Last week the Constitutional Court blocked an investigation into whether to strip the president of immunity from prosecution.

What’s needed, and is unlikely to happen under Mr. Pérez Molina, is real reform: banning private funding for political parties, guarantees of equal media access and the establishment of quotas for candidates who are indigenous people or women. Absent reform, elections would hand-deliver a mandate to the opposition and usher in another four years of Guatemalan politics controlled by a criminal mafia. The only real chance for such reform lies with a minority in Congress, outsider candidates for election and the demonstrators themselves.

Guatemalans have long historical memories. Their vision of the United States continues to be shaped by the C.I.A.-sponsored coup in 1954 that interrupted an initial democratic spring and triggered decades of armed conflict. After years of painstaking efforts to repair relations between the United States and Guatemalan society, Guatemalans are angrily pointing their fingers at the United States, which they see as either propping up an increasingly illegitimate Guatemalan government or treading with excessive caution, refraining from using its substantial influence to move democracy forward.

Second chances rarely happen. Yet the United States is being offered one now. By publicly aligning itself with the diverse coalition of Guatemalan citizens seeking immediate democratic reforms, the United States has an opportunity to bolster a democracy that Guatemalans deserve and lay the foundation for a constructive relationship with an emerging Guatemalan political class. In helping regenerate a Guatemalan democratic spring, this time the United States can unequivocally stand on the right side of history.