Caroline Lucas, the Green party’s only MP, has declared she will stand for the party’s leadership in a job share with its work and pensions spokesman, Jonathan Bartley, under the slogan “The Power of Working Together”.

The pair say they want to forge a new “progressive alliance” with other political parties willing to advocate electoral reform – potentially including deals over who would contest particular parliamentary seats.

“You’d be looking at a number of seats where there would be a whole variety of things you could do,” said Lucas. “I really think there is an appetite out there now for a less tribal politics.”

Lucas, who led the Greens before the current leader, Natalie Bennett, took over in 2012, is the party’s best-known face. The MP for Brighton Pavilion since 2010, she was immediately touted as a likely successor when Bennett said she would not stand again when her term expires at the end of August.

Lucas added that the idea of a job share appealed because it would allow her to keep up with her job as a constituency MP – and bringing more voices into politics is a key part of the Greens’ platform.

“Because of the responsibilities I still have as a constituency MP and a voice at Westminster, taking on the job of leader on my own was not something that would sit well with that – so it was fantastic to have the conversation with Jonathan,” she said.

“There is a sense, which we felt very strongly with the job share idea itself, that more ideas, the more plural your politics, the more likely they will be effective, and reach more people”.

Bartley stood against Labour MP Chuka Umunna in the Streatham constituency in the 2015 general election.

He told the Guardian he believed Labour’s shift to the left under Jeremy Corbyn’s leadership had created a “once-in-a-generation opportunity to form a progressive alliance to bring people together, and actually change politics for a generation”.

Lucas said Labour’s fragile electoral position, with just one MP left in Scotland after last year’s general election, meant it would need to strike new agreements in order to govern. She added that key figures in the party such as John McDonnell had said they were in favour of electoral reform.

“The writing’s on the wall that whoever is the leader of the Labour party, they are going to have a Herculean job to win an outright majority at the next election. What we believe is that that is a huge opportunity to crack open the current political system.”

Under Bennett, the Green party expanded rapidly, and now has 60,000 members – not far behind the LibDems’ 67,000. But it failed to make the hoped-for electoral breakthrough at last year’s general election, despite her appearance in the televised leaders’ debates.

Lucas and Bartley say they would hope to professionalise the party – creating a team of full-time campaigners who could help out in different constituencies at election times, for example.

Bartley said he was drawn into the Green party after the experience of caring for his son, Samuel, who is a wheelchair user. “Samuel’s life to me has been a political statement, and has profoundly changed my outlook on the world,” he said.

He described the moment his son took part in a school sports day, rolling towards the finish line in his electric wheelchair when other children had already crossed the line. “People started chanting his name and they cheered him across the line. At that point, in that school, everyone knew that it really was the taking part that mattered and not winning, and the presence of someone who is slightly different, vulnerable, changed the values in that school.”