Alabama governor Robert Bentley resigned on Monday rather than face impeachment and pleaded guilty to two misdemeanour campaign violations which arose during an investigation of his alleged affair with a top aide.

In a remarkable fall from grace, the 74-year-old Republican and one-time Baptist deacon stepped down as the sex-tinged scandal gathered force over the past few days. Legislators turned up the pressure by opening impeachment hearings on Monday.

Last week, the Alabama Ethics Commission cited evidence that Bentley broke state ethics and campaign laws and referred the matter to prosecutors.

"There have been times that I let you and our people down and I'm sorry for that," Bentley told Alabama's State Capitol chamber after he pleaded guilty.

The violations were discovered during the investigation of his affair but were not directly related to it.

In court, Bentley appeared sullen and looked down at the floor. One misdemeanour charge against Bentley stemmed from a $50,000 loan he made to his campaign in November which investigators said he failed to report until January. State law says that major contributions should be reported within a few days.

The other charge stemmed from his use of campaign funds to pay nearly $9,000 in legal bills for political adviser Rebekah Caldwell Mason last year.