A tweet mentioned during the final presidential debate on RTÉ was an “earth shattering” moment for Seán Gallagher and may have changed the course of the election, a member of his campaign team claimed today.

Richard Moore, who was Mr Gallagher’s press advisor during his unsuccessful bid for the Áras, said the tweet read out by host Pat Kenny on the Frontline programme led the candidate to falter on live television.

He said Mr Gallagher's performance, just days before polling, resulted in a major fall-off in support. Until then, he was well ahead of his main rival, the eventual winner Michael D Higgins, in opinion polls.

Mr Gallagher’s team is to lodge a formal complaint later this week with both the Broadcasting Authority of Ireland (BCI) and the RTÉ Authority over the handling of the debate.

The tweet in question was mentioned by Kenny shortly after Mr Gallagher had responded to a claim by Sinn Féin candidate Martin McGuinness that he had collected a €5,000 cheque from a businessman and convicted fuel smuggler for a Fianna Fáil fund-raiser in Co Louth in 2008.

“[There is] a development which I want to put to Seán Gallagher,” Kenny said. “On the @Martin McGuinness4President Twitter account, Sinn Féin say they will produce the man who gave you a cheque for €5,000.”

It later transpired the tweet did not come from the official McGuinness campaign account or Sinn Féin.

Speaking this morning, Mr Moore said RTÉ should have checked before mentioning the tweet on live television.

“The level of caution and care you have to enter into in relation to live [presidential] debate should be a bit more than, well, here’s a tweet let’s throw it at the candidate,” he said. "RTÉ might have to look at the question: did they treat him fairly?”

Mr Moore said he would be "flabbergasted" if the broadcaster did not adopt a policy on social media in the coming months. In the past, people "of a vexatious nature" who wished to get into the media had to negotiate a "frontline defence" of journalists who would be able to check their veracity, he said. “That onus isn’t there any more,” he claimed.

He said the tweet “spooked” Mr Gallagher. “He was very much aware of the fact that he hadn’t handed over any money but it did just cast a doubt in his head.”

Mr Moore said the candidate then appeared to falter a bit. “Before 900,000 viewers on a live TV show, that can be earth shattering,” he said. “And it was earth shattering. There’s no doubt about it but that it was – not the only reason – but probably the principal reason as to why apparently so many people deserted his campaign in the last 48 hours.”

An opinion poll conducted on the day of the election showed a significant swing from Mr Gallagher to Mr Higgins, with a total of 35 per cent of people questioned saying they were influenced a lot by the Frontline debate.

Mr Moore also said the complaint may also raise the time allocated to each candidate during the debate.

“There was a specific focus by Frontline on Seán Gallagher, presumably because he was 15 points clear [in the opinion polls]. But under broadcasting rules, it’s not good enough to simply dance your way around that one,” he said.

“Even if someone is 15 points clear (in the polls) it doesn’t mean you can just devote most of the programme and most of the 10-mark questions to the one candidate.”

The formal complaint, which is still being finalised, will be submitted this week. Under current regulations, complaints must be lodged with 30 days of an election.