Mr Cameron said too much power would go to Brussels under the treaty
The Conservatives will not let the government "off the hook" on the European constitution, Tory leader David Cameron has insisted.
A bill tabled by Mr Cameron would allow for a referendum on the Lisbon Treaty.
He promised a referendum if he took office before all EU states ratified it, but declined to say what he would do if this happened before a Tory win.
The public could "pile the pressure" for a public vote by electing Conservative MEPs, he told the BBC.
Under the bill introduced by the Conservatives, a referendum would be held on the same day that the Republic of Ireland voted for the second time on whether or not to ratify.
The treaty was ratified by the UK in June 2008, but it does not come into force until it has been approved by all member states.
As well as the Irish referendum in the autumn, the Czech Republic and Poland have yet to confirm ratification and there is a legal challenge to German ratification.
Mr Cameron told BBC Radio 4's Today programme that too much power would pass from London to Brussels under the treaty, and promised he would "not let matters rest" if elected.
But he would not be drawn on what he would do if he arrived in Downing Street after all EU states had completed the ratification process.
"I don't want to go into every last detail of what happens if a series of things happen - if there isn't an early election, if the Irish vote yes in a second referendum, if the Poles decide to ratify this treaty, if the Czechs decide to ratify," said Mr Cameron.
"I know that, of course, my opponents would love me to focus on what happens if all these things happen, but I am not going to do that.
"On Thursday, people can go into those voting booths, vote Conservative and pile the pressure on Gordon Brown to hold a referendum. I don't want to let him off the hook."
Mr Cameron added that the Conservatives believed in British membership of the EU, but wanted it to be about trade and co-operation "rather than this endless process of building a superstate".
He also repeated his commitment to withdrawing the Conservatives from the European Parliament's European People's Party (EPP) and setting up a new centre-right bloc.
Former Tory cabinet ministers and European commissioners Lord Brittan and Lord Patten have strongly criticised the decision, which they say will reduce the Conservatives' influence.
But Mr Cameron said he had held talks with the Czech Civil Democrats and Poland's Law and Justice Party - both parties of government - and said the new group would better represent the UK's interests in Europe.
"It will bring to European politics a centre-right group of parties that are committed to reform in Europe and change in Europe - it won't just be the British Conservatives, it'll be other parties as well," Mr Cameron said.