The inclusion of the party would help to split the left-wing vote and harm Labour and the Lib Dems.
The party is determined to deny Clegg the chance to make a fresh appeal.
Rather than preparing for another hung parliament, Labour is focused on "crushing" Clegg's party.
Labour has a 16-point lead over the Tories, with the Lib Dems in fifth place behind Ukip and the Greens.
Cable has previously suggested that the coalition could end early, a demand now made by the Lib Dem peer.
With the general election result so uncertain, leadership contenders in the Tories, Labour and the Lib Dems are making themselves known.
Party writes to Jeremy Heywood asking whether Coetzee's involvement in Lib Dem election strategy has breached the special advisers' code of conduct.
The rise of UKIP and the fall of the Lib Dems explains why Labour is maintaining a lead approaching the final year of this parliament.
Rather than Jeremy Browne, the party needs a centre-left figurehead, such as Tim Farron, to revive its fortunes.
How many know that the party still aspires to abolish tuition fees and plans to review the bedroom tax?