Representative Henry Cuellar, a Texas Democrat in a crucial swing district, and his wife were charged with participating in a yearslong $600,000 bribery scheme involving Azerbaijan and a Mexican bank, according to a federal indictment unsealed in Houston on Friday.

The accusations against Mr. Cuellar, 68, and his wife Imelda, 67, center on allegations of bribery and money laundering in connection with their efforts on behalf of an oil and gas company owned by Azerbaijan’s leaders as well as an unnamed bank based in Mexico City, according to the 54-page complaint.

Mr. Cuellar, a Laredo native first elected in 2004, is also accused of acting as an agent of a foreign entity while a U.S. government official — by delivering a speech favoring Azerbaijan in Congress and inserting provisions into aid bills to benefit those who were paying bribes to his family.

The government claimed that Mr. Cuellar, who once served as Texas secretary of state, was paid to back legislation intended to stymie regulation of the payday loan industry, which has been accused of predatory lending practices against the poor. He also tried to weaken money laundering laws that affected Mexico’s banking sector, according to prosecutors.

Mr. Cuellar “agreed to influence legislative activity and to advise and pressure high-ranking U.S. executive branch officials regarding measures beneficial to the bank,” they said.

Many of Mr. Cuellar’s actions, taken at face value, appeared to be legitimate policymaking activities, albeit favoring foreign-owned businesses. But the indictment claimed hidden motives, and prosecutors compiled tables of secret payments that cast those actions as an illegal and mercenary scheme.