Jo Swinson has ruled out backing a Jeremy Corbyn-led government after a general election, saying the Liberal Democrats could not even have an informal support arrangement with Labour, despite recent cooperation to block a no-deal Brexit.

Speaking ahead of her first Lib Dem conference as party leader in Bournemouth on Sunday, Swinson said she favoured “pluralist politics” and cross-party work, such as when opposition MPs grouped together to force a Brexit extension and block an election.

But major policy differences and serious personal worries about Corbyn and Boris Johnson meant she could not support either in government, Swinson told the Guardian.

With the Lib Dems buoyed by a recent wave of MPs arriving from other parties, and more defections predicted, Swinson cast the upcoming election as a choice between the old-fashioned, Brexit-minded two main parties and her party’s outward-looking stance.

“The faultline in British politics is no longer a left-right divide, and that is what the Labour-Conservative duopoly of politics was predicated on,” said Swinson, who became the first woman to lead the Lib Dems in July.

“The fundamental faultline now is between liberal and authoritarian values, and as Liberal Democrats we are clearly on one side of that. Both Labour and the Conservatives are having to straddle that faultline. That is why those two parties are creaking at the seams.”

Swinson accused Corbyn of “totally, totally failing” to eradicate antisemitism from Labour, and said Johnson viewed politics as “a game”.

The faultline in British politics is no longer a left-right divide, and that is what the Labour-Conservative duopoly of politics was predicated on

“In Boris Johnson’s case I just do not think he cares about people,” she said. “I think he only cares about himself. I find it easy to get on with most people in politics, but I find that difficult to respect.”

Swinson has sought to distinguish the Lib Dems’ remain message from Labour’s by saying her party would revoke article 50 and cancel Brexit without a referendum. She conceded some would view this as “a bold move”, but argued it would be anomalous for a Lib Dem government to be obliged to negotiate a deal in Brussels when it was so publicly wedded to staying in the EU.

A post-election hung parliament could see Swinson cast as the powerbroker, potentially able to put Corbyn or Johnson into Downing Street. Neither would happen, she said. “Jeremy Corbyn is not fit to be prime minister. Boris Johnson is not fit to be prime minister. On a daily basis we get evidence of both of those things.”

Swinson dismissed the possibility of supporting Corbyn along the lines of the Democratic Unionist party’s confidence-and-supply arrangement with the Conservatives. “I don’t know how I can be clearer – I do not think he is fit to be prime minister. He is trying to take our country into Brexit. He has failed to tackle antisemitism in his own party.”

Lib Dem numbers in the Commons have swelled in recent weeks as Chuka Umunna, who moved to the party shortly before Swinson became leader, was joined by fellow ex-Labour MPs Luciana Berger and Angela Smith, as well as the former Tories Phillip Lee and Sarah Wollaston and the byelection winner Jane Dodds.

The new arrivals have caused internal tensions, with some Lib Dem activists complaining about “celebrity MP entryism” after Umunna and Berger were lined up for winnable London seats.

Swinson said an influx of MPs was “a nice problem to have”, but conceded it could result in some existing candidates being asked to stand down: “That has to be managed, but I’ve been really impressed at how people have responded to that.”

The arrival of Lee, who abstained in the vote on equal marriage and argued that immigrants should show they do not have HIV or hepatitis B before being allowed into the UK, has caused a separate row, with the head of the Lib Dems’ LGBT group quitting in protest.

Swinson conceded the move had caused difficulties but argued this was mainly one of messaging, defending Lee for “a very nuanced position”, which, she said, had been misrepresented in parts of the media. “Obviously, I regret that some people have been upset by that. I think we can certainly learn some lessons about our internal processes, how we communicate.”

She stressed the party would not accept any MPs without question, saying would-be new entrants must undergo a 90-minute grilling from the Lib Dem chief whip, Alistair Carmichael, to “make sure they share our liberal values”.

Recent weeks have illustrated the paradox faced by Swinson ahead of a conference at which the Lib Dems will celebrate record membership of more than 120,000: while Brexit is the party’s great foe, it is also arguably the prime reason for its surge.

“If you gave me a choice I’d put no Brexit above having a good opportunity for the Lib Dems, every day of the week,” she said. “But this is a huge opportunity, which is obviously exciting. I want to see a liberal society, and there’s more possibility now that we might be able to achieve that. It does also feel like a very critical and perilous time for our country. The stakes are very high.”