Clare Hilley used her victory in the Waddon byelection to attack the negative tactics used by the opposition.
In her victory speech, Councillor Hilley, who enjoyed a 1 per cent swing in the vote, said under Tony Newman's leadership “Labour really is the nasty party”.
The 25-year-old, who secured 1,462 votes, said: “I have fought against Ian Payne [the Labour candidate] before and it has been a clean fight.
“Unfortunately on this occasion he allowed Tony Newman to dictate the campaign and as a result it has been completely negative.”
Watch the Waddon Byelection result
Leader of the council Mike Fisher added: “This shows people are turning off negative campaigning.
“It is a very good result and a very good candidate.”
Coun Newman said he knew what Coun Hilley was referring to but stood firm in his tactics.
He said: “We will see much more on that debate over the next 12 months and I look forward to that debate.”
Coun Newman said he was disappointed in the result and conceded it was “not good enough tonight”.
Accepting her post, Coun Hilley paid tribute to the work of Coun Jonathan Driver, whose death on New Year's Eve prompted the byelection, saying he would be a hard act to follow.
She also promised to build the promised swimming pool in Waddon and and vowed to deliver her pre-byelection pledges.
She added: “I am going to be a dedicated councillor.”