Let's examine one race, the one in South Dakota, where an MD, Annette Bosworth, is running for the Senate against former Governor Mike Rounds, a candidate that has his own scandal(s) brewing, one of which involves a federal investigation and a dead witness.
Annette Bosworth ran into a lot of trouble with South Dakota medical authorities a while back for not providing proper care. Essentially, she's not a practicing doctor anymore, but one who hangs out a non-profit health care shingle so she can develop various scams. She's engaged in a series of scams involving "lotteries" where people pay her money for a chance to purchase farmland, the profits of which she claims will go to fund various causes, including veterans causes. But, apparently, most of these "lotteries" have not been conducted under the law, and most of the charities have not seen any money.
The employees running her non-profit scam have bailed, claiming she refused to pay their wages, and they are suing.
Many in the know in South Dakota think "Dr." Bosworth is in the race in order to dilute the ultra-conservative vote so that Mike Rounds, ethically challenged as he is, can slide through a primary against two other candidates.
I don't know how many other Doctors in races are as "f-ed" up as "Dr." Bosworth's, but I wouldn't assume all these docs are in it to win it. For Annette, it may be a way to scam some money and do the establishment candidate a favor.