The Lib Dems could back David Cameron's plans for an In/Out referendum on EU membership in 2017, if they are allowed to set the question and other concessions.
The party has suggested that it could be persuaded to support a vote, if issues such as allowing EU nationals to take part were ironed out.
The move would likely increase the chances of another Lib Dem-Tory coalition after May.
Previously Mr Cameron has pledged to hold a vote only if he has a majority in Commons after the election.
Sir Malcolm Bruce, the Lib Dem deputy leader, said that his would not agree to a "a willy-nilly referendum with no context".
But he added: "If there is to be a referendum, we certainly think that young people should have the vote.
"We would like them to have the vote in all elections, as they did in the Scottish referendum, so we would certainly legislate for that.
"As far as European citizens are concerned, they live here, they work here, they are paying their taxes here.
"And they are on the electoral register. Frankly, it seems to be that it would be unfair, and probably unconstitutional, to deny them the right to vote in any such referendum.
Patrick O'Flynn, the Ukip MEP, said that including EU nationals would be unconstitutional was "nonsense".
"This referendum is a chance for British people to vote on how they are governed and by whom and I think we are seeing the lid lifted on the way establishment politics is done in this country.
"We know from very well-connected journalists ... that David Cameron is plotting coalition two.
"And now we are beginning to see what Nick Clegg's conditions are.
"We are beginning to see that they are planning a rigged referendum, changing the franchise, allowing people with a huge vested interest to vote".
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