Though unlikely to increase their number of MPs in the General Election, Plaid Cymru are among the parties - including the SNP and the Greens - who have formed an anti-austerity alliance, and may just be the king-makers in a hung parliament.
But what do the party stand for and who is their leader Leanne Wood?
So, who is she?
The first female leader of Plaid Cymru.
Born in the Rhondda, where she still lives today, she was elected as the South Wales Central region representative in the National Assembly in 2003. She is the first Plaid party leader not to be a native speaker of the Welsh language.
How did she come to be elected leader?
A surprise winner in some ways, Leanne Wood was elected in March 2012, comfortably defeating the favourite Elin Jones (who is now Plaid Cymru’s Shadow Minister for Health and Well-being) and Dafydd Elis Thomas - more of him later.
ITV Leaders' Debate: Who's in it?
ITV Leaders' Debate: Who's in it?
1/6 ITV Leaders' Debate
Tory Leader David Cameron will take the podium (appropriately) on the far right
2/6 ITV Leaders' Debate
Labour leader Ed Miliband will stand on the centre-right podium
3/6 ITV Leaders' Debate
Ukip leader Nigel Farage will take the centre-left podium
4/6 ITV Leaders' Debate
SNP leader Nicola Sturgeon will stand at the podium second furthest on the right
5/6 ITV Leaders' Debate
Appropriately enough Green Party leader Natalie Bennett will take the furthest-left podium
JUSTIN TALLIS/AFP/Getty Images
6/6 ITV Leaders' Debate
Plaid Cymru leader Leanne Wood will stand on the centre-right podium
What does she believe?
Ms Wood is an outspoken socialist, republican, feminist and advocate of Welsh independence - making her quite unusual in British politics.
In 2012, prior to her election as party leader she told the Independent: ‘I’m a socialist, I recognise the world is not an equal place, I support measures to equalise things.’
What about her family life?
She is intensely private. Has a long-term partner and a daughter Cerys Amelia.
Where did she go to school?
She was educated at Tonypandy Comprehensive School (now Tonypandy Community College), and the University of Glamorgan.
Has she had any jobs outside politics?
Prior to her election to the Welsh Assembly she was a probation officer and co-Chair of the National Association of Probation Officers. She also worked as a lecturer at Cardiff University.
Any controversies then?
Ahem. Quite a few actually.
Wood was famously the first member of the Welsh Assembly to be asked to withdraw from the chamber.
During a debate on the Queen's Speech in December 2004 she launched a fierce attack on the planned policy of introducing ID cards and new anti-terror laws.
In a passionate speech she slammed the policies saying they came "from a Labour government you should all be ashamed of".
During the debate she referred to the Queen as "Mrs Windsor" and was asked to withdraw the reference by the then Presiding Officer, Lord Elis-Thomas, on the grounds that it was discourteous.
She refused and was asked to leave the chamber. She later told the press: ‘I don’t recognise the Queen’ and ‘I called her that because that’s her name.’
She was also arrested in 2007 for protesting against the UK's Trident nuclear missile programme at Faslane naval base in Scotland. Plaid have repeatedly stressed that Trident is a major stumbling block to them supporting a minority Labour government.
What does Plaid Cymru actually stand for?
Plaid's manifesto, which was in the press for the wrong reasons earlier this week, is centred on securing parity of powers and resources for Wales with Scotland, ending austerity and 'permanently rebalancing the UK economy with targeted investment'.
Leanne Wood and the party strongly believe Wales is hindered by not being able to control levers that affect the Welsh economy. The party also advocates a living wage for all employees by 2020.
Among their more controversial policies is the plan to fund an extra 1000 doctors for the Welsh NHS using a tax on fizzy drinks.
They say: "We will fight for Wales every day and secure equality for our country with the other nations of the UK so we have the tools and the resources to unleash Wales’ economic potential and work towards world-class public services."
Number of followers?
Not really one for Twitter jokes. Plays it very straight. Has a very loyal following.
Plaid Cymru are extremely active on social networks so expect to see tweets praising Wood in your timeline.
What might we not know about her already?
She takes her inspiration from the 1976 novel 'Woman on the Edge of Time', by Marge Piercy.
Ms Wood has said reading it was “as though someone had removed a blindfold”.
She is strongly anti-war, blaming George Bush and Tony Blair for home-grown terrorism.
She backs the decriminalisation of cannabis and has called for heroin addicts to be prescribed the drug.
She is pro-immigration and once slept on the streets of Cardiff to highlight the issue.
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