MOST Coalition MPs back the war in Afghanistan but with little optimism about the chance of victory, a Liberal senator says.

''We can't win it in any meaningful sense of the word, but we can hope that we can achieve a measure of stability,'' Queensland senator Russell Trood said.

He denied the parliamentary debate this week on the nine-year war amounted to a Clayton's contest, given there was no specific motion to withdraw troops. Senator Trood, a former international relations academic, said it was a bad idea to give Parliament a veto over sending troops to war.

''I think governments have to be responsible for putting Australians at risk in the field,'' he said. ''They do it with a level of intelligence knowledge that most of us in the Parliament do not have access to.''

Greens MP for Melbourne Adam Bandt said Parliament should debate any deployment of troops. He said although it was hugely distressing for the families of the 21 Australian soldiers killed in Afghanistan, community views on the conflict should be aired.

''One has to ask what the ongoing rationale for deployment is,'' Mr Bandt said. ''It can't be to fight the Taliban, because we now know the Afghan government is having discussions about power-sharing with the Taliban. It can't be to fight al-Qaeda, because analysts are now saying al-Qaeda is operating from elsewhere.''

The debate will run from tomorrow to Thursday, and will be held in the Senate next week.

Opposition Leader Tony Abbott said Coalition members were free to speak their mind in the debate. ''We've never had a Stalinist approach to these issues. I'd expect Coalition members to say what is in their heart,'' he told Nine Network. He said after he saw a demonstration of firepower during his recent Afghanistan visit, he was convinced ''there is certainly no lack of firepower available''.

''I'm not saying there mightn't be ways of further assisting [but] what we need to do this week is … win the hearts and minds of Australians for the Afghan mission,'' he said.

Labor left-winger Laurie Ferguson said one reason the left wasn't more concerned about the commitment was that the alternative to the present regime would be worse, for example on women's rights, but negotiations with the Taliban were inevitable.