The candidate they had planned to back, Femi Oluwole, told the BBC he had pulled out over concerns his candidacy would hand victory to the Brexit Party and ultimately harm the campaign for another referendum.
He said the WTO (World Trade Organization) no-deal option argued for by Brexit Party leader Nigel Farage was a “weak” option and “needs to be exposed”.
The case against it, though, “hasn’t been made as strongly as it should have been”.
“The only two countries that operate effectively under WTO rules are China and Russia,” he said.
The EU is “far from perfect” and there are “many, many things that need changing”, but it “does provide security”, Mr Cable added.
The Green Party has said there are “fundamental ideological differences” between the Remain parties on other issues.
Chris Leslie, the former Labour MP who quit to join Change UK, has previously told Business Insider he didn’t think an alliance would “ever be likely because we are starting something new”.
Labour ‘all over’
Mr Cable is due to stand down as Lib Dem leader this month.
The party’s education spokeswoman, Layla Moran, ruled herself out of the running to succeed him on Tuesday, saying her “first priority has to be to serve my constituents to the best of my ability” and being leader would dilute that effort.
Deputy leader Jo Swinson, the frontrunner to replace Mr Cable, criticised Labour for “negotiating to try to make Brexit happen” and for being “all over the place” on the issue.
Her comments come as talks between Labour and the government continue talks aimed at finding a way to leave the EU.
At the same time, Labour deputy leader Tom Watson says his is the party of “Remain and reform”, while Brexit spokesman Sir Keir Starmer has said there should be a referendum on any cross-party deal.
Who is standing in the European elections?
The UK is divided into 12 regions, each represented by between three and 10 MEPs depending on population size.
Seats in England, Scotland and Wales are awarded to parties according to their share of the vote, to candidates on lists drawn up by the parties.