A row over abandoning Britain's nuclear deterrent will pose a serious challenge to Sir Menzies Campbell's leadership of the Liberal Democrats at the party's spring conference.
Activists have mounted a serious challenge to his policy of putting off a decision on replacing Trident until 2014.
And, despite the insistence of close aides that there was "no massive ideological division" between the two sides, any defeat would be seen as highly damaging.
The issue - over which the third party's leadership suffered a major reverse in 1986 - is set to dominate the party gathering in Harrogate.
The Government has already announced that it will press ahead with renewing the UK's nuclear deterrent with a new generation of nuclear submarines.
While he has vowed to oppose the "premature" decision in a Commons vote later this month, Sir Menzies' official policy is to cut the missiles by half now but put off a long-term decision.
Opposition will be centred on a bid to take an immediate decision not to renew but to keep the deterrent in place for the rest of its lifetime - around 20 years.
The amendment - which will include senior MP Phil Willis among its backers - also criticises the closeness with the US fostered by reliance on the Trident system.
And it calls for the money saved to be spent on much-needed equipment for conventional armed forces.
The leader's chief of staff Ed Davey insisted that the leadership remained confident of winning the vote.
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