Anthony Bays, aged 55 years, turned to forgery to try to get extra Conservative votes during an election experiment in Guildford, Surrey.
The council was trying out a "postal votes only" system during last May’s local government elections in England and Bays thought up the idea of collecting ballot papers before voters returned them in the post.
Yesterday Judge Jonathan Playford, sitting at Reading Crown Court, heard that the former councillor had decided to change his plea to three counts of forgery to guilty.
Bays had been due to face trial by jury after he initially denied forging ballot papers during the Guildford Borough Council elections.
The defendant, who had been a borough councillor in Lambeth for more than 25 years, was arrested just two weeks after he won his seat. He subsequently resigned.
Judge Playford heard how Bays, now a train ticket inspector, had offered to deliver ballot papers when he knocked on the door of an elderly couple in his ward.
Although the 80-year-old woman was reluctant, she handed over her form, on which she had voted for a Liberal Democrat candidate - and her husband’s, which was blank because he was unable to vote.
Bays, a former accountant, opened the envelopes containing the ballot papers in front of the pensioner, before delivering them to the civic offices later that afternoon on 1 May.
However, a receptionist, who knew that candidates were not supposed to handle ballot papers, alerted the returning officer, who took them from Bays.
The officer then received a phone call from the pensioner later that day, expressing her concerns, which prompted him to look at the confiscated papers.
He saw that the woman’s vote had been tampered with - Anthony Bays’ name and another Conservative candidate had been added to the form.
Her husband’s form had also been written on, with three Conservative candidates nominated and the signature forged.
Paul Cavin, prosecuting, said: "At about 4pm on 1 May this defendant approached the receptionist in the borough offices and was clearly holding some ballot papers and asked where he should deposit them.
"All the election agents had been warned that under no circumstances should any candidates handle ballot papers.
"This was something that concerned the receptionist who called the deputy returning officer, and the papers were taken off the defendant.
"He claimed to be delivering them for voters who couldn’t deliver them. That was something he shouldn’t have done.
"Later that day election officers received a phone call from a concerned voter. She had seen the defendant earlier that day and he had knocked on her door and asked her whether she had voted yet. She hadn’t, and neither had her husband, who was living in a home.
"She handed over two ballot papers - one for herself, on which she had voted Liberal Democrat, and one for her husband which said ‘return to sender’."
Mr Cavin then described how the pensioner saw Bays opening the envelopes in front of her, which prompted her to call the returning officer at the council, who went to examine the confiscated ballot papers he had taken off Bays earlier that day.
A recount delivered the seat to Liberal Democrat power.
Sentencing was adjourned until April so that pre-sentence reports could be prepared.