Sen. Luther Strange of Alabama has a presidential endorsement and multimillion-dollar backing from establishment Republicans, but he can't shake suspicion about the circumstances of his appointment or speculation about whether he's under criminal investigation.

Strange is facing a stiff primary challenge from former judge Roy Moore, and President Trump will attempt to tip the balance campaigning with Strange on Friday, days ahead of the Tuesday vote.

The former state attorney general was appointed to the Senate in February by then-Alabama Gov. Robert Bentley as his office investigated Bentley during a complex scandal involving a mistress.

Opponents allege a corrupt trade between Strange and Bentley, who resigned as part of an April plea deal, and speculation abounds about whether authorities are working to determine what actually occurred.

"I heard Monday there is an active investigation," Republican state Rep. Ed Henry told the Washington Examiner. He declined to identify his source, but said he heard it was a state rather than federal probe.

Henry claims Bentley told him he selected Strange "to get rid of him" because "he's corrupt." The former governor denies the conversation happened. Henry recently has declined to comment on whether authorities interviewed him about the alleged conversation, citing his attorney's advice.

"I have to be very careful about what I say," Henry told the Washington Examiner.

Another Strange opponent, Republican state Rep. Mike Ball, told the Washington Examiner "there is plenty of evidence that gives me reason to believe that former Attorney General Strange and others could have committed crimes."

"If there are no state and federal investigations, the proper authorities are asleep at the wheel," said Ball, chairman of the House Ethics and Campaign Finance Committee.

Law enforcement offices that could be investigating Strange — the FBI, the local U.S. attorney's office, the Alabama attorney general's office and the Montgomery district attorney — aren't commenting, leaving a fog unlikely to clear before ballots are cast.

Strange spokesman Cameron Foster said the senator has not been questioned by federal or state investigators related to the appointment.

Bentley attorney Bill Athanas said the ex-governor has not been questioned about the appointment since leaving office in April.

Both Strange and Bentley deny anything untoward took place.

There are publicly known reviews of the appointment, but they either are incomplete or their status is unknown.

The Alabama Ethics Commission, which has the power to recommend prosecution, as it did against Bentley, has not taken any official action on a complaint regarding the appointment, said commission executive director Tom Albritton.

A spokesman for the Alabama State Bar, meanwhile, said he was unable to comment on the status of a complaint seeking Strange's disbarment for unethical conduct.

The controversy over Strange's appointment existed even before he interviewed with Bentley for the job because he was widely believed to be investigating the governor.

Days before the November election, Strange requested that the state legislature pause impeachment proceedings against Bentley pending "related work" by his office. When Trump opened Jeff Sessions' Senate seat by making the Alabama Republican attorney general, Strange sought the job and refused to say if Bentley was under investigation, calling speculation "unfair" to Bentley.

By accepting the Senate seat, Strange allowed the governor to appoint a new state attorney general to investigate him. Notes written by Bentley and released this week show he found the "trickle-down effect" appealing.

Bentley's pick to replace Strange, however, immediately acknowledged the investigation, recused himself, and named Ellen Brooks special prosecutor. Brooks negotiated a plea deal with misdemeanors in exchange for Bentley's resignation.

Brooks said in a cryptic April statement that the plea deal "concludes the investigation as to the former governor, but does not necessarily conclude the investigation."

A spokesman for the attorney general's office, Mike Lewis, declined on behalf of Brooks to comment on whether her work continues.

A spokeswoman for Moore, best known nationally for defying federal court orders on a Ten Commandments monument and same-sex marriage, did not respond to a request for comment.

The issue is one of many in a race whose main narrative is establishment versus outsider. But it is contentious nonetheless.

"We will get to the truth about Luther Strange's appointment deal with Governor Bentley while he was under investigation by the Attorney General's office," Moore said in a Tuesday statement about Bentley's notes.

Some Alabama Republicans aren't sure what to make of allegations and insinuations.

"I think it's important to know if a candidate for any public office is under investigation," said Republican state Rep. Allen Farley. But, he added, it's also important also to understand whether political retribution prompted a probe.

Democrats, meanwhile, are feeling upbeat about a Dec. 12 general election where victory should be unthinkable. "We have a real shot at winning this Special Election!!" Democrat Doug Jones' campaign said in a Wednesday fundraising appeal.