The deputy Liberal Democrat leader has dismissed historic tweets from their newest member Chuka Umunna, in which he attacked the party.
After Mr Umunna, a Labour-turned-Change UK MP, announced he would be joining the Liberal Democrats, his old posts criticising them reemerged.
One of the tweets posted in 2013 read: "You can't trust a word the Lib Dems say".
Another accused them of "betraying students" over tuition fees.
But Jo Swinson told talkRADIO’s Mike Graham Mr Umunna's past was not important.
“This isn’t about the past, this is about how do we solve the problems that our country is facing now,” she said.
Tweets from Mr Umunna slating the Liberal Democrats have reemerged
Mr Umunna left Labour in February to become a Change UK MP, but then defected to the Liberal Democrats after poor European election results.
Following his latest defection, Labour has called for a by-election to be held in his Streatham constituency.
On Twitter, Labour Party chairman Ian Lavery wrote: “Put your immense popularity to the good people of Streatham...let's have a PV (People's Vote) on you and your principles.”
Ms Swinson argued that “people are elected on the ballot paper as an individual” and another vote was not required.
The Liberal Democrat leadership hopeful said Mr Umunna has acted with “absolute good faith”.
“It’s hard for people when that happens to come to that conclusion and to leave a party. Joining a different party, that’s an additional hurdle for people to get over mentally in their mind,” she said.
“Chuka with absolute good faith thought that what the country needed was a new party and tried to make that work.”
She added: “What we know it that is it very difficult, particularly with our electoral system, to set up a new party in British politics.”