Jo Swinson is to use her first conference speech as Liberal Democrat leader to argue the “tired old parties” are losing their dominance following failures over Brexit, amid claims that the Lib Dems could take 100 or more seats at an election.

Swinson is due to close the gathering in Bournemouth on Tuesday afternoon with a rallying cry to activists ahead of a likely imminent election, and an appeal to voters via renewed condemnations of Boris Johnson and Jeremy Corbyn.

“The tired old parties have failed, looking inward at a time of national crisis,” she is expected to say, according to a speech extract released in advance. “Our country needs us at this precarious time. We do not have 10 or 15 years. We need to seize the opportunity now.”

Following his own speech to the conference on Monday, the party’s foreign affairs spokesman, Chuka Umunna – among six MPs to join the Lib Dems from Labour or the Conservatives since the summer – said an election could bring significant gains.

The party has 18 MPs, but is polling at up to 20%. Umunna told reporters: “I would hope that we get more than 40 seats at a general election.

“But we know from the internal polling that if we move from the position that we’re in, and say there is a 1.5% to 2% swing, we can get up to 100 seats, and if there’s a 5% swing towards the Liberal Democrats through the course of the campaign 200 seats are in contention – but who knows what will happen?

“We are not complacent, we are not drunk on our success, we want to continue to expand the bandwidth, to draw people into the party.”

Other senior Lib Dem figures have talked privately about expectations for an election ranging from a base of about 40 seats to up to 100, with the party optimistic that its new Brexit stance will attract voters.

Earlier at the conference, members voted to endorse Swinson’s proposed policy that the party would revoke Brexit without a referendum, albeit only in the unlikely event it won an absolute Commons majority.

The plan has encountered some internal opposition, with one Lib Dem MP, Sir Norman Lamb, saying he was worried such a proposal could stoke divisions. “I think that the polarisation that we are seeing is incredibly dangerous,” he told the BBC. “I think we are playing with fire in many ways.”

Swinson has pledged not to support either Johnson or Corbyn in power, arguing that neither are fit to be prime minister.

She is expected to further attack both in her leader’s speech, saying Johnson’s plans to mitigate a no-deal Brexit would be “like planning to burn your house down – you might have insurance, but you’re still going to lose all your stuff”.

Swinson also condemned the PM for suspending parliament, ejecting 21 Conservative MPs who rebelled over a backbench law mandating him to seek an extension to Brexit, and then hinting he could ignore that law.

She said: “Silencing critics, purging opponents, ignoring the law – for someone who proclaims to hate socialist dictators, he’s doing a pretty good impression of one.”

Similarly, Swinson’s speech criticised Corbyn for what she called a lukewarm attitude to staying in the EU: “If he had campaigned to remain in 2016 with half of the energy he put into the 2017 election, we may have seen a different result.”

In his speech to the conference on Monday, Umunna argued that Johnson’s threats to break the law meant Britain was unable to exert any moral authority in a world increasingly dominated by authoritarian leaders.

The Streatham MP said the globe faced an increasing challenge from illiberal and autocratic leaders such as Donald Trump, Vladimir Putin, Viktor Orbán in Hungary and Turkey’s Recep Tayyip Erdoğan.

“A giant battle is raging between the pluralist creed of liberal democracy on one hand, and a desiccated authoritarianism on the other,” Umunna said.

“What is clear is that we will not see the leadership on the world stage required from the new occupant of No 10.”

“You cannot defend a liberal, rules-based order when you so openly flout the rules at home. Boris Johnson has facilitated the takeover of Her Majesty’s government by the remnants of the Vote Leave campaign.”