Green Party leader Caroline Lucas has pledged her support for a change in the voting system, saying a switch to the Alternative Vote would be "liberating".
Ms Lucas, who became the party's first MP last year, said moving away from first-past-the-post would "keep MPs on their toes" and curb tactical voting.
The party has long argued for a different, proportional voting system.
Speaking later at the party's Spring conference, she said her party was the "real opposition" to the coalition.
Ahead of the speech in Cardiff, Ms Lucas - MP for Brighton Pavilion and leader of the party in England and Wales - said she believed only the Greens could stand up against the government's deficit reduction plan, its proposed overhaul of the NHS and the sell-off of the Royal Mail.
"Labour is utterly contaminated by the fact they pursued many of the policies which now the coalition government is simply taking to their logical conclusions, so I think people are looking for a genuine opposition - and you won't be surprised to know I think that's the Green Party," she told BBC Radio 4's Today programme.
"I've always been a bit ambitious, but I do think the Green Party is the real opposition."
Despite long campaigning for MPs to be elected through a system of proportional representation, the Greens are backing a switch to AV - a non-proportional system in which voters rank candidates in order of preference - in May's referendum.
Under the current system, she said too many MPs represented "safe seats" - historically dominated by a single party - and this was "hugely damaging in terms of the complacency they almost breed".
Moving to AV would mean people did not "have to agonise over tactical voting," she added.
"They can vote for what they believe in and I think that will be very liberating for a lot of people."
The cross-party No to AV campaign said Ms Lucas had had a "Damascene conversion" to AV.
"Perhaps Ms Lucas would like to answer why she has changed her mind and is now telling voters to support a voting system that will cost the taxpayer £250m and which she described as "embarrassing" and "fake" just a year ago," a spokesman for the campaign said.
Launching their official campaign last week, supporters of the current system argued that switching to AV would be "costly and complex" and warned the system could produce "unfair" results - all claims contested by their Yes to Fairer Votes opponents.