Labour has selected author and satirist John O'Farrell as its candidate to fight the Eastleigh by-election.
He is the last of the main party candidates to be picked for the contest on February 28, which was triggered when Liberal Democrat ex-Cabinet minister Chris Huhne resigned in disgrace.
The Lib Dems are favourite to retain the seat, which Mr Huhne secured at the 2010 general election with a majority of 3,864. Local councillor Mike Thornton is representing the party.
However, the Tories have signalled they are setting aside coalition links to mount a fierce campaign. They have selected Maria Hutchings, a staunch eurosceptic and opponent of David Cameron's gay marriage policy.
Ukip candidate Diane James on Tuesday said Romanians were associated with crime and called for a temporary halt to all immigration. Ms James was in the Hampshire constituency for a rally with supporters aided by Ukip leader Nigel Farage.
Ms James, from Ewhurst in Surrey, said: "Immigration is the elephant in the room that the other parties want not to address because they are heavily reliant on unrestricted immigration."
When asked what she meant, Ms James said: "Because in the last few years immigrants have settled in areas where there are a large number of Labour voters.
"On the January 1 2014 the floodgates will open for Bulgarian and Romanian citizens (to come to Britain). We are not just talking about pressure on services from immigration but also, and I have to say it, the crime associated with Romanians.
"One of the problems with the Olympics last year was the Metropolitan Police having to deal with Romanian criminal gangs pick-pocketing."
She said that unrestricted immigration was an issue for the country. "Until we sort out the UK Border Agency and make it fit for purpose, the only way is a moratorium on immigration," she said.