There are currently some 9,000 British troops in Afghanistan
Lib Dem leader Nick Clegg has warned that the mission in Afghanistan is "failing" and said a "completely new strategy" needed to be drawn up.
He said there was "one last chance" to do that, telling the BBC the current effort was "unfair on our brave British servicemen and servicewomen".
And ex-leader Lord Ashdown warned that pulling troops out would give al-Qaeda free rein and endanger UK security.
They spoke ahead of a vote by Lib Dems on how to end the UK's Afghan mission.
Members agreed on Tuesday morning to support a motion calling for a "focus on concluding the Afghan mission".
The motion was drawn up by grassroots members, but a party spokesman said the leadership was "happy" about it as it "sent a strong signal" about the need for a change of strategy.
While stopping short of calling for a timetable for the withdrawal of the UK's 9,000 troops - in Afghanistan as part of a 40,000 strong Nato force - the motion says ministers must report to Parliament "on progress towards a withdrawal".
It also backs moves towards a regional peace process which would involve negotiating with elements of the Taliban.
The top US commander in Afghanistan, General Stanley McChrystal, has said the mission risks failure unless more troops are sent.
But when asked whether the Lib Dems believed the UK should back US calls for reinforcements, a party spokesman said the focus should be on a "political surge".
This was echoed by the wording of the motion which called for an end to the "military first" approach.
Mr Clegg said: "What we're in favour of is a completely new strategy and we've got one last chance to do that because what we're doing at the moment in Afghanistan is failing and that's unfair on our brave servicemen and servicewomen."
'Sacrifice and pain'
Lord Ashdown, once blocked by the Afghan government from taking up a senior diplomatic role in the country, has been increasingly critical of the Nato mission, saying its aims have become blurred.
He said there had never been a coordinated plan of action drawn up by all countries acting in Afghanistan.
"Unless we get a strategy, a united international strategy, an interlocking political and military one, no amount of extra troops are going to win this battle," he said.
Lord Ashdown also called on Gordon Brown to do more to unite the British public behind the Afghan mission.
"He has completely failed to explain why our troops are there," Lord Ashdown said. "We're not a squeamish people, we can take sacrifice and pain if we're convinced we know what the war is for and there is a reasonable prospect of success.
"Both of those things have been absent for the last three or four years.
"I think there is a real possibility now that we will lose the battle in Afghanistan in the pubs and front rooms of Britain before we lose it in the deserts and mountains of Afghanistan."
Speaking earlier at a fringe meeting, Lord Ashdown said the repercussions of military defeat or forced withdrawal from Afghanistan would be disastrous for the region and the world as a whole.
While al-Qaeda was currently confined to North-West Pakistan, the exit of foreign troops from Afghanistan would give the terrorist group "the whole country to play around in".
This would increase the security threat to the UK since there was a direct link between "Osama Bin Laden in his cave to a terrorist flat in Bolton".
Lord Ashdown said failure in the Afghanistan would harm the UK
The withdrawal of troops could irreparably undermine the Nato alliance while proving a body blow to Pakistan, whose internal "collapse" it could trigger.
It would also damage efforts to support moderate Muslim societies around the world, he added.
"Failure or withdrawal would lead to consequences that are baleful," he said. "I don't think these are consequences that we should seek."
Separately, Lord Ashdown said the party had to have "serious answers" to an unprecedented combination of domestic and global crises if it was to continue to prosper.
As the only party with a truly "internationalist" outlook, it had to "shout louder" about the merits of continuing to fund foreign aid in a recession and the need to support international institutions.