Tulchan and Davidson, the former leader of the Scottish Conservatives, received heavy criticism last week after she was hired as a part-time senior advisor. The MSP for Edinburgh Central, who resigned her party leadership role in August, is due to step down from the Scottish Parliament in 2021.
Tulchan, which is not a member of the PRCA, insisted that Davidson will not carry out any lobbying activities in her new role.
But PRCA director-general Francis Ingham said it was "simply wrong for lobbying agencies to employ legislators". He said the "possible conflict of interest in doing so is clear" and the move "damages the reputation of both our industry, and of the political process".
Writing in an opinion column in PRWeek, Grant said there's a "deep conflict between what the PRCA says and how it acts as a 'regulator'".
"Its statement is unequivocal and states that 'PRCA members are prohibited from employing parliamentarians'. However, a brief look at the PRCA Board shows that Steffan Williams’ firm Portland employs the former minister Baron James O’Shaughnessy. Ed Williams has Lord Myners as the chairman of Edelman UK. Teneo is listed as a member of the PRCA and they have William Hague, Baron Hague of Richmond, as a senior advisor.
"Try as I might I can find no press release from the PRCA condemning these roles and appointments."
Grant added: "I think these firms should employ these peers, they are all men of great wisdom and high integrity, but I also think the PRCA are wrong to make the public criticism of Tulchan."
In response, Ingham said: "Andrew is simply wrong. The rules of the PRCA Public Affairs Code are clear, and the companies he cites abide by them fully, and always have done.
"There is a clear difference between peers occupying internal advisory positions, and elected politicians working in client-facing roles.
"The peers Andrew names have internal advisory roles at board-level; they are contractually prevented from undertaking any lobbying work; and - crucially - they have no engagement with clients.
"That is not the case with Tulchan’s appointment of Ruth Davidson. As their statement welcoming her says, they 'have no doubt that our clients will benefit immensely from her insight'.
"It remains our clear judgement that for a lobbying firm to appoint a currently-sitting, elected MSP to a client-facing role is simply unconscionable. Ruth Davidson should choose between being an MSP and being a lobbyist. She cannot be both."
A spokesperson for Edelman said: "Paul Myners does not advise clients nor does he do public affairs work for Edelman. He is a cross-bench peer who chairs Edelman’s Advisory Board, which gives strategic advice and provides external challenge to the UK management team. This is entirely within the PRCA rules."