Jo Swinson has told Lib Dem members there is no point being “narrow and pure”, following a series of defections to the party by former Tory and Labour MPs.
The Lib Dems have been boosted by six new MPs in recent weeks, with former Conservative minister Sam Gyimah the latest to be unveiled.
However the influx of MPs has led some to question if the party’s core principles will be diluted.
Some Lib Dem LGBT activists quit their roles after ex-Tory MP Phillip Lee joined the party, citing his previous call for immigrants to be tested for HIV.
But speaking to the Lib Dem conference in Bournemouth on Tuesday, Swinson said the party had to be the “rallying point” for the liberal centre-ground.
“The opportunity in front of us is huge, and it is for the taking,” she said.
“We can win. We must win. And to do so we must build the biggest liberal movement this country has ever seen.
“We cannot be satisfied with a place on the fringes of British politics, narrow and pure, small and irrelevant.”
“Our job is to gather forces of liberalism and be the rallying point for change.”
Swinson has moved to shore up the party’s anti-Brexit credentials with a new promise to revoke Article 50 on “day one” should the party win the general election.
“Today I’m standing here as your candidate for prime minister,” she told Lib Dem members on Tuesday.
The party currently has 18 MPs – boosted by defections – meaning it would need more than 300 additional seats to secure a majority.
In her speech, her first as party leader, Swinson also took aim at Prime Minister Boris Johnson.
“Boris Johnson’s insults of choice are rather revealing – big girl’s blouse, girly swot,” she said.
“But let me tell you, conference, if he thinks being a woman is somehow a weakness, he’s about to find out: it is not.”