The Liberal Democrats, Plaid Cymru and the Green Party announced they have formed a "Remain Alliance" which will see them not stand against each other in a number of seats in the upcoming General Election.
As part of the Unite to Remain pact in England and Wales, the Lib Dems will stand in 43 constituencies, the Greens will stand in 10 and Plaid Cymru will stand in seven.
Speaking at the Unite to Remain press conference, Heidi Allen said millions of people in the UK are looking for "an alternative way".
She said targeting 60 "key" seats will "make all the difference" at the General Election.
"This isn't a normal General Election, anybody that tells you it is, is not being truthful," Ms Allen added.
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Ms Allen said: "This is a General Election that's all about Brexit whether we are going to leave and whatever Boris Johnson manages to get through or whether we have an opportunity to stay."
She continued: "And these parties recognise this, it's not about them, it's not about how many seats they're going to win, it's about what is best for the country."
Lib Dem leader Jo Swinson said: "This is an election where the issue of Brexit is at its heart.
"We are making that decision about the future of our country and it is important that in an election like this, we are doing everything we can to maximise the chance that we can remain in the European Union," she added.
When asked why the Unite to Remain alliance does not include the Labour Party, Ms Swinson said: "Labour did not want to engage in this process, perhaps the clue is in the name, this is Unite to Remain, Labour by their own admission is not a remain party."
She said the Labour Party has not been involved in discussions because of their unclear stance on Brexit.
She said: "We've had Shadow Cabinet members talking about how they would campaign to leave the EU and Jeremy Corbyn can't give a straight answer to the question of whether he is remain or leave.
She continued: "So that is why I think Labour have refused to engage with Unite to Remain and, we as Liberal Democrats have worked constructively with the Greens and Plaid Cymru who have been happy to have those conversations."
During the election campaign, Ms Swinson has categorically ruled out working with Jeremy Corbyn, even if it meant another general election.
The agreement between the parties comes after success in the Brecon and Radnorshire by-election, which saw the Liberal Democrats win the seat from the Conservatives after the other two parties stood aside.
Commenting on the parties coming together, Plaid Cymru's Liz Saville-Roberts said the "easiest thing under the sun" for the parties to do would be to "go back to the comforts of tribalism".
Ms Saville-Roberts said she hopes the parties coming together gives "some credibility" to politics.
"Our message is clear. This is what matters to us and this is why we're acting this way," she added.
Green MEP Molly Scott Cato told the Unite to Remain press conference that she hopes today will be seen as a "historic day", and she hopes the deal will "break the two-party stranglehold".