Ed Miliband is to appeal for Labour voters to pass up the temptation to damage Nick Clegg and support reform of the electoral system.
The Alternative Vote referendum should not be reduced to a verdict on the Deputy Prime Minister's "broken promises", Mr Miliband is to insist.
The intervention, in a speech to the Labour Yes campaign in Westminster, comes just days after the party leader told Mr Clegg he was so unpopular he should lie low in the run-up to the May 5 poll.
Mr Miliband will say he wants a Yes result because "politicians should never feel safe or insulated from those they represent".
"Let's be honest. AV is no panacea. It isn't perfect," he is to admit. "But it would help to restore the balance of power in favour of voters.
"So on May 5 ask yourselves one simple question - are you happy with the state of British politics?
"If the answer is no, then seize this opportunity for change."
He will say the campaign is about more than "whether a Yes vote damages David Cameron or a No vote damages Nick Clegg".
"I want to take head-on the fear designed to appeal to Labour supporters: that a yes vote in this referendum will be seen as a vindication of Nick Clegg," Mr Miliband will add.
"I know this referendum is far harder to win because of Nick Clegg's broken promises. But we can't reduce the second referendum in British political history to a verdict on one man. The change to the alternative vote deserves our support because it is fairer and because it encourages a better politics."
