Jenna Ellis, a lawyer who played a major role in the efforts to keep Donald J. Trump in power after his 2020 election loss, reached a cooperation agreement on Monday as part of a deal with prosecutors leading an election interference case in Arizona.

She is the first of the 18 defendants in the case to reach such an agreement. Ms. Ellis already pleaded guilty to a felony last year in a similar case in Georgia. In Arizona, nine felony charges against her were dropped in exchange for her cooperation.

“This agreement represents a significant step forward in our case,” Kris Mayes, the Arizona attorney general, said in a statement. “I am grateful to Ms. Ellis for her cooperation with our investigation and prosecution. Her insights are invaluable and will greatly aid the state in proving its case in court. As I stated when the initial charges were announced, I will not allow American democracy to be undermined — it is far too important.”

This is a developing story. Check back for updates.