At some point during his first term as an assemblyman, Eric A. Stevenson made a portentous decision: He decided “to sell official actions in exchange for thousands of dollars in cash,” a federal prosecutor told a Manhattan jury on Tuesday.

The prosecutor, Brian A. Jacobs, said that Mr. Stevenson, 47, had taken more than $20,000 in bribes to help four businessmen build an adult day care center in his district in the South Bronx.

Mr. Stevenson, a Democrat, then introduced legislation that would have made it illegal for competing centers to open in New York City for three years, a step worth hundreds of thousands of dollars to the businessmen who had bribed him, Mr. Jacobs said.

“Why did Mr. Stevenson do all these things? It’s simple: money and greed,” Mr. Jacobs said in an opening statement in Mr. Stevenson’s trial on corruption charges in Federal District Court.