The Alliance of Democrats and Liberals for Europe could now move up from being the fourth largest to the second largest party in the parliament.
The Alliance of Democrats and Liberals for Europe (ALDE) has set out its policies at its annual congress in Madrid, ahead of the European Parliamentary elections in May 2019.
As it did so En Marche, the party of French President Emmanuel Macron, also announced it would join ALDE - potentially making it the second largest bloc in the EU parliament, up from fourth.
ALDE presented itself as an alternative to recover the "soul" and the essential values of Europe against the threats of populism and nationalism.
In the manifesto it launched ALDE said: "The choice is the following: we update the European Union to develop individual freedom, prosperity and stability, or we go back to the times of nationalism and growing authoritarianism."
It also outlined positions on immigration, climate change, Brexit, terrorism and other challenges facing Europe.
"The authoritarian, nationalist and populist movements within the EU are trying to undermine" the fundamental values of the European project, according to ALDE, which warns that these anti-liberal forces are pushing Europe towards a "cultural conflict".
On Brexit it called on "the EU and the United Kingdom to make every effort to avoid a scenario of 'no agreement' that would have negative consequences for all involved and for Ireland in particular. "
"If the United Kingdom decides to revoke its decision to leave the EU, we will welcome that decision and work for the restoration of a renewed and stable relationship," it added.
The UK's Liberal Democrat party, which has almost no chance of winning an election, is promoting the idea of holding a second referendum.
According to opinion polls would another referendum would now be won by Remainers, many of whom are now supporting the idea. Prime Minister Theresa May has however always ruled it out.
“We think that there is a good chance that we will be able to build up a majority to have a referendum, a people’s vote as we call it, with the option of remaining in the European Union,” its leader Vince Cable said at the congress.