The Green Party has called on Labour to join it in withdrawing candidates from an Oxfordshire constituency in a bid to defeat Tory health minister Nicola Blackwood.
The party wants to allow the Liberal Democrats to stand largely unopposed against the Conservative as she seeks re-election in Oxford West and Abingdon.
The Lib Dems have already stood aside in Brighton Pavilion for Green Party co-leader Caroline Lucas as the broadly left-wing parties attempt to do deals to thwart the Conservatives.
Sarah Wood, chairwoman of the Oxfordshire Green Party, said: "Our political system is broken, and it makes no sense that parties with many common values stand against each other and let the Tories through."
Labour rejected a similar deal for the Isle of Wight, despite Greens standing down for Labour's Rupa Huq in London, although the likes of Labour's Clive Lewis have voiced support for these alliances.
The attempt to prize the Oxford West and Abingdon seat from the Conservatives will require a significant voter swing.
After claiming her Oxfordshire seat by a mere 176 votes in 2010, Ms Blackwood increased her majority to almost 10,000 in 2015's election.