Layla Moran, the Liberal Democrat MP competing to become the party’s next leader, has castigated its tactics at the last election, saying a pledge to revoke Brexit lost it the trust of voters, while Jo Swinson’s talk of becoming prime minister was not seen as credible.

Moran said the Lib Dem’s national image was “broken” and the party had to reconnect with voters and examine what it stood for after Brexit, or else risk “bumping along with 10% support and 10 or 11 MPs for a number of years”.

The Oxford West and Abingdon MP also said that whoever becomes the new Labour leader must end the “very tribal politics” between the parties and cooperate to the extent of electoral pacts.

“I think we’re quite a way off from that right now,” said Moran, the party’s education spokeswoman, who announced her candidacy just over a week ago. “Nevertheless, I do think that Labour need to look themselves in the eye and decide if they want to win or not.”

Her most pointed comment concerned the Lib Dems’ hugely disappointing election campaign, during which the then leader, Swinson, initially presented herself as a prime minister-in-waiting, only to lose her seat as the party slipped to 11 MPs.

Moran – who stresses she takes her share of responsibility for the decisions – said the ambitious stance was initially justified, but should have been changed after the Brexit party stood down the bulk of its candidates in favour of the Conservatives.

“Then, under the first past the post system it actually became not just unlikely, but frankly impossible that we were going to form a government, and that lost us credibility,” she said.

Moran, who is travelling the country talking to Lib Dem activists and to voters about what went wrong, also criticised the leadership’s shift ahead of the party’s conference last autumn to a policy of revoking Brexit without a second referendum, albeit only in the unlikely situation of winning an absolute majority.

There was at the time, she said, “a fear that Labour was going to roll its tanks on to our turf and we needed something to be distinctive”. But, she argued, the idea was pushed through the party without proper debate.

“I’ve been talking to activists who say, yes, they were bounced into it, partly because the decision was made in rooms in Westminster with not enough dissenting voices before,” Moran said. “So by the time it went to conference, it felt like it was a confidence motion.”

While the Lib Dems pride themselves on being a grassroots-led party, Moran said this was being lost: “I’m concerned, if you look at the parliamentary party makeup at the moment, we are going to be perceived as a party of the elite of London and the south-east.

“And that absolutely is not what I want for our party, we need to be a party for the entire country. And so, yes, we absolutely need to change the way we make policy.”

Layla Moran visits a school with Jo Swinson during the 2019 general election campaign. Photograph: Dan Kitwood/Getty Images

The two biggest obstacles the party faced in December’s election, Moran said, remained the electoral system and potential voters’ distaste for Jeremy Corbyn. “But that doesn’t absolve us from our own responsibility for decisions that we took,” she said.

Moran refused to say which of the three candidates to succeed Corbyn she would most like to work with, but said that whoever did take over would need to not just cooperate better with other parties and embrace electoral change.

“They need to appreciate that unless they work with us to change the electoral system it’s possible that the Conservatives, particularly with the lack of strength of Labour in Scotland, are going to have a majority forever,” she said. “They need to get quite pragmatic about that, in my view.”

The only other candidate to formally announce they are running for the Lib Dem leadership so far is Wera Hobhouse, the MP for Bath, although Ed Davey, who took over as interim leader after Swinson lost her seat, seems certain to do so as well. Daisy Cooper, who won St Albans for the party in December, is also considering joining the race.

Nominations do not open until 1 May, with voting among members not completed before mid-July.

Moran said a key issue was developing new policy priorities, saying she had asked voters what they knew about the Lib Dems: “Unfortunately, for many of them, the answer is nothing. With Brexit, they knew what we were against. But when you ask them, well, what are we for, they don’t have an answer.”

Her three priorities would be to boost equality of opportunity via education, tackle the climate emergency, and work towards more collaborative politics.

The Lib Dems, she said, still had a strong local base, with popular councillors. “It’s the national brand that’s broken. And in my view, that’s what needs to change.”