Llyr Powell, who is standing for Reform in an upcoming Welsh Parliament by-election, told ITV Cymru Wales the admissions of Nathan Gill were “abhorrent”.
Gill was a member of the Welsh Parliament between 2016 and 2017 and leader of Reform UK in Wales for a short period in 2021.
He pleaded guilty on September 26 to accepting bribes in exchange for making pro-Russian statements while he was a member of the European Parliament.
But Mr Powell has come under scrutiny for his own links to Gill, having worked as his constituency caseworker while he was an MEP.
Speaking at First Minister’s Questions on Tuesday, Baroness Eluned Morgan said: “This is really serious stuff and I do think there are questions to answer by people, not least by Reform’s candidate in the Caerphilly by-election, Llyr Powell, who was employed as an adviser to Gill.
“I think we need to know, what did he know, when did he know it and did he report it?”
Gill, 52, of Anglesey, North Wales, pleaded guilty at the Old Bailey to eight counts of bribery on dates between December 6 2018 and July 18 2019.
His activities were said to include making pro-Russian statements about events in Ukraine in the European Parliament and in opinion pieces to news outlets.
Mr Powell told ITV Cymru Wales: “It’s an abhorrent thing to do, to take money off a foreign state when you’re an elected member there to represent the people.
“I found it a complete and utter betrayal myself, and I can understand why voters are angry at Nathan on this.
“It’s deeply upsetting to hear and see that someone who called himself a patriot betrayed us in that way.”
Gill was first elected as a Ukip member of the European Parliament in 2014 and his role ended when the UK left the EU in 2020, at which point he was an MEP for the Brexit Party.
He led Reform UK’s 2021 Welsh Parliament election campaign but is understood to have had no involvement with the party since.
Mr Powell says he stopped working for Gill in 2017, before the offences took place.
In light of the comments made by the First Minister, Mr Powell, the Reform candidate in the upcoming by-election in Caerphilly, accused Labour of a “desperate smear campaign”.
He added: “I want them to be clear on what they’re trying to allude to in this and be clear because I have said nothing or done nothing that warrants these sort of smear campaigns.
“This by-election isn’t easy, I can tell you I’ve put up with a lot.
“I’ve had the death threats. I’ve been assaulted.
“But I can’t tell you if they think any of this is going to prevent me in continuing on what I believe is the right thing to do here.”
