The trial of Missouri Governor Eric Greitens over a sex scandal was scheduled to begin on Thursday as the Republican politician faced mounting pressure to resign.

Jury selection was due to get under way in a state circuit court in St. Louis, and opening statements were expected to begin early next week in a case that has state lawmakers weighing impeachment.

Greitens is charged with felony invasion of privacy in connection with an admitted extramarital affair in 2015 before his election. He has said he is innocent of any criminal wrongdoing and called the relationship in question consensual.

He has vowed to remain in office while he fights to clear his name.

Greitens, 44, is accused of taking a photo of his lover in a state of undress without her consent and making it accessible by computer to use as retaliation should she divulge their relationship. He has denied threatening to blackmail her.

Woman alleges blackmail threat

The alleged offence occurred in March 2015, the year before Greitens, a married father of two and former U.S. Navy SEAL commando, was elected governor. If convicted, he faces up to four years in prison.

The verdict could come down to the technical workings of the iPhone, the definition of "transmission," and the whereabouts of a photo that may or may not exist.

The backlash against Greitens, a onetime Republican Party rising star, grew after he was charged in a separate case with computer tampering. Prosecutors allege he obtained and transmitted a donor list from a military veterans charity he founded in 2007 without the charity's consent to aid his political fund-raising.

The Republican-controlled Missouri legislature will convene a special session on May 18 to consider impeachment or other discipline. No Missouri governor has ever been impeached.

Greitens has cast himself as the victim of a "political witch hunt" in both cases, which were brought by Kim Gardner, the Democratic Circuit Attorney for the city of St. Louis.

The woman testified to state lawmakers she believes the picture was taken while she was bound, blindfolded and partially nude in Greitens's basement.

The woman — a hairdresser whose name is expected to be released during trial after a judge's ruling on Wednesday — told investigators she saw a flash through the blindfold and heard what sounded like a photo being taken.

Greitens allegedly told her, "You're never going to mention my name, otherwise there will be pictures of [you] everywhere."

Transferring a photo can lead to prosecution

The woman said that she became upset and that Greitens told her he deleted the picture. Prosecutors acknowledged in court Monday that they have not found such a photo. And Greitens has repeatedly declined to say if he took a picture.

The woman described in a state legislative report released last month a tumultuous, months-long affair punctuated by instances of physical abuse, jealous rage and manipulative behaviour by Greitens.

The scandal may tarnish the Republican Party and dim its hopes of ousting Senator Claire McCaskill, a Democrat regarded as one of the more politically vulnerable senators in the upcoming November congressional elections.

Under Missouri law, simply taking an unauthorized photo of someone in a full or partial state of nudity is a misdemeanour, punishable by up to a year behind bars. It is a felony carrying up to four years in prison if the image is distributed to someone else or if a person "transmits the image in a manner that allows access to that image via computer."

There is no evidence Greitens shared or posted any photo. At Monday's court hearing, prosecutor Robert Steele said authorities planned to call an expert who would explain that "transmission from the pixels to the CPU, or the memory card, is a transmission." That would mean that any cellphone photo involves transmission.

On Tuesday, a forensics examiner was at the courthouse extracting data from Greitens's phone. Michelle Richardson of the Washington-based Center for Democracy and Technology said in an interview with Associated Press that even if a photo is deleted, it isn't erased from the phone's memory and can be retrieved.

Many phones are also set to automatically back up photos to the cloud, storing data on the internet through places like Apple's iCloud or Google Photos. Phone users can opt out of storing data to the cloud, but many don't.

Prosecutors could seek a search warrant against Apple or Google to try to find the photo. It's unclear if they have done so.