AMs are not meant to use assembly facilities for party-political campaigning.
But in March, South Wales Central AM Neil McEvoy discussed the campaign at a press conference held in an assembly briefing room.
Mr McEvoy accepted that he was in breach of the AM's code of conduct.
The standards committee has recommended to the assembly that Mr McEvoy be censured for the breach - a formal reprimand.
Their conclusion follows an investigation by the standards commissioner Sir Roderick Evans who said Mr McEvoy "failed to comply" with the rules on 14 March.
The press conference was held while Mr McEvoy was suspended from his party group in March - a suspension that was later lifted.
In a statement, Mr McEvoy said he distributed copies of a document at the press conference which contained seven policies relevant to the local elections, saying they were relevant to his suspension.
He said: "This was not appropriate. I accept that it was in breach of the code of conduct."
A Plaid Cymru spokeswoman said: "This is a matter for the individual member who apologised and we are pleased that this matter has been resolved."

Cumhuriyet newspaper's editor-in-chief Can Dundar and its Ankara representative Erdem Gul have been charged with espionage.
Prosecutors accuse them of working with a US-based cleric to discredit the government.
The harsh punishment being sought has intensified press freedom concerns.
EU Enlargement Commissioner Johannes Hahn said he was "shocked" at the severity of the sentence sought by prosecutors.
Human Rights Watch said the two "were doing their job as journalists and no more than that".
In its report last May, Cumhuriyet published video of police finding weapons in trucks that it said were linked to Turkish intelligence.
The Turkish authorities insisted the trucks, which had been intercepted near the Syrian border, were in fact bringing aid to Syria's Turkmen minority.
But the report caused uproar and prompted President Reccep Tayyip Erdogan to file a lawsuit against the journalists.
Mr Erdogan said the video footage was a state secret and vowed on TV that the journalists "would pay a heavy price".
The pair were detained in November and told the BBC they were kept in solitary confinement for 40 days before being allowed to share a cell.
The government accuses them of helping the Hizmet movement led by Fethullah Gulen, who lives in self-imposed exile in Pennsylvania.
Mr Erdogan has accused Mr Gulen of plotting against him, although Mr Gulen denies this.
Mr Dundar and Mr Gul are accused of working with Hizmet to create the impression that the Turkish government was helping terror groups, thus weakening its ability to rule.
They face charges of espionage, attempting to overthrow the government and support for a terror organisation, Hurriyet reported.
If convicted, they will receive an "aggravated life sentence", which includes tougher conditions and restricted leisure hours, the Dogan news agency reported.
Turkey has come under mounting criticism for its treatment of journalists.
Last week, US Vice-President Joe Biden called on Turkey to protect freedom of expression during a visit to the country and also met with Mr Dundar's wife in a show of support.
Press freedom 'a major concern'
A Plaid Cymru AM has apologised for discussing his party's council election campaign at a press conference in the Senedd building.
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Two Turkish journalists face life in prison over a story alleging that the Turkish government was arming Islamist militants in Syria.