Liberal Democrat leader Tim Farron has described himself as a "bit of a Eurosceptic" despite his party's pro-EU stance.
Mr Farron cited his 2008 resignation from the Lib Dem front bench because he wanted a referendum on the Lisbon Treaty.
But he said only the Lib Dems could prevent the "calamity of hard Brexit".
The party is aiming to attract the votes of people who backed Remain in June's EU referendum.
Speaking on the BBC's Andrew Marr Show, he made a pitch for voters in former Lib Dem heartlands in the South West, where many people voted to leave the European Union.
"I don't want to go off on a little bit of a rabbit hole here, but you will remember that I resigned from the Liberal Democrat front bench about 10 years ago because I am a bit of a Eurosceptic," he said.
"I'm somebody who challenges people in power - the EU, in government, in councils - but I am somebody who believes Britain is better off in the European Union."
He also said he admired ex-prime minister Tony Blair for his electoral success, and claimed that Labour was now too divided to focus on the election.
Reiterating that there would be no Lib Dem coalition with Labour, he said a Conservative majority after the 8 June general election was "not in question" and that Mrs May was heading for a "colossal coronation" - making questions about party pacts meaningless.
"We're determined to turn the coronation she expects into a contest that Britain desperately needs," he added.
Labour said the Lib Dems could not be trusted, pointing to the party's promise not to raise tuition fees - a stance it abandoned in coalition government.