Two members of the Llanelli Plaid Cymru branch have been suspended.
Gwyn Hopkins and Meilyr Hughes were among 26 local members who accused Mari Arthur of rule breaches, including withholding a set of office keys.
Cardiff-based Ms Arthur became the candidate after Plaid centrally ruled the winner of a local hustings vote was not eligible as it was a target seat.
She has been asked to comment.
One of the suspended members, Mr Hopkins, has been a Plaid Cymru councillor for 28 years.
Sean Rees won the local vote for the Plaid nomination in April and is among the Llanelli members who have asked for a "high level investigation" into Ms Arthur's conduct.
Since the election, when Ms Arthur came third, members say Mr Rees, who is also the local press officer, was blocked from a party Twitter account and forced to work from a library.
They also claim that a "secret group" has been created on Facebook by Ms Arthur's campaign group calling for the "getting rid of the dead wood" from the Llanelli constituency committee.
The complaining members say: "We are very concerned with the current situation in Llanelli and ask for your intervention in fully investigating all of the unconstitutional actions of the constituency secretary, Ms Arthur."
In response to a compliant about the selection process, Plaid Cymru's chairman Alun Ffred Jones said: "He [Sean Rees] is very young and doesn't possess professional qualifications as far as I know, with no experience except as an organiser in Llanelli."
Mr Rees was offered the opportunity to try for the Swansea West or Gower nomination.
Mr Rees said he "had always worked in the interests of the party locally and nationally".
Plaid Cymru said: "The sharing of private information regarding the party's internal disciplinary process is a matter which Plaid Cymru takes extremely seriously."
A party spokesman added: "Some individuals are understandably disappointed by selection outcomes and this occurs in all political parties.
"The party is confident these matters will be resolved according to our usual procedures."
It is understood that mediators will try to resolve the dispute.