The Liberal Democrats are on a mission to go from "protest back to power", the party's departing leader, Sir Vince Cable, has said.
In a speech in York, Sir Vince called for the party to continue arguing for the benefits of staying in the EU.
He also accused Prime Minister Theresa May of prioritising Conservative Party unity over maintaining peace in Northern Ireland.
Sir Vince, 75, will step down in May after leading the Lib Dems since 2017.
'We are Remain'
Speaking on Sunday at the party's spring conference, Sir Vince said "we are Remain", adding: "Whatever happens in the next few weeks of parliamentary twists and turns, we must argue - since no-one else can be relied upon to do so - that none of the several mutually exclusive versions of Brexit on offer - soft or hard - are as good as the deal we currently have."
Next week, Mrs May is expected to bring her withdrawal agreement back to the Commons for a third time after it was twice voted down by large margins.
Mrs May's efforts to win over Tory Eurosceptics to back the deal have focused on attempts to revise the backstop, the measures in the Brexit deal aimed at preventing the return of a hard border in Ireland.
"The intensity of the campaign to remove it speaks volumes about the underlying motives of those who demanded Brexit and now demand a 'clear Brexit'," Sir Vince said.
"They simply deny our history, which is entwined with that of Ireland."
Sir Vince also targeted Northern Ireland Secretary Karen Bradley personally for criticism, following a series of gaffes.
Ms Bradley previously said that deaths caused by the security forces in Northern Ireland during the Troubles were "not crimes" - comments she ended up apologising for.
"It really is quite shocking that this government is so lacking in talent that it employs a secretary of state for Northern Ireland who says she doesn't understand sectarian voting patterns and then compounds this public declaration of ignorance with a blatantly and naively one-sided view of the killings in the Troubles," Sir Vince said.
"Ms Bradley has revealed an ugly truth: that peace in Ireland matters less than peace in the Conservative Party."
Sir Vince, who clashed repeatedly with Mrs May over immigration policy while they sat around the Cabinet table during the coalition years, used his speech to return to the issue, saying it highlights a divide in British politics.
"Our mission to move from survival to success, from protest back to power, takes place in a world where liberal values are under siege and in retreat.
"Nothing quite defines liberalism like its opposite, illustrated by Theresa May's policies on immigration."
The Lib Dems have 11 MPs - down from the 57 they had in 2010.
The party has struggled electorally since 2010, when it formed a coalition government with the Conservatives.
Sir Vince, a former business secretary under the Coalition government, will step down after the English local elections in May.
Leading candidates to replace him include the current deputy leader, Jo Swinson, relative newcomer Layla Moran and former environment secretary Ed Davey.