Two Labour senators joined forces with ex-Fianna Fáil minister Mary O’Rourke to urge voters to save the upper house, as Mr Gilmore tried to play down the breach of party discipline.
Senator John Whelan accused the Government of attempting to grab more power for itself via abolition.
“The party’s policy originally was to put this to the constitutional convention. I am in favour of reform, but the Government doesn’t want a strong Seanad, they just want to get rid of it so that they vest more power in the hands of a few ministers and their faceless advisers who are on €158,000 a year — which is three times more than a senator gets.
“I don’t know any senator on the Government side who is in favour of abolition,” he told RTÉ.
“We are asking voters to save themselves from a powerful elite.”
The senator said the referendum would cost taxpayers €20m and should have been delayed as, whatever the outcome, the Seanad will serve its full term.
Mr Gilmore insisted that Ireland did not need “two parliaments” and said all parties had differences of opinions over referendums, but all Labour TDs and senators had voted to hold the national poll.
“In all political parties there are individuals, public representatives, who have a view that is different to the official view of the party. This is something we’ve had before,” the Tánaiste said.
Mr Gilmore insisted that a reformed Dáil could take over any checking role currently enjoyed by the Seanad.
The Labour leader dismissed claims abolition would amount to a “power grab” as it was part of a process of democratic reform.