Lose the vote on EMU and Britain will be semi-detached from Europe for the foreseeable future, consigned to the periphery of a dynamic economic power and cut off from the political leverage enjoyed by our neighbours. Sterling will be impossibly volatile, and the euro will be adopted by those who can afford it as a parallel currency.
Lose the vote on electoral reform and Britain will be stuck with a half- reformed, half-democratic constitution, prey to the advance of a reconstructed and reactionary Conservative party.
Lose either of these votes and the programme of modernisation and renewal ushered in with the defeat of the Conservatives on 1 May 1997 will remain unfinished business - probably for an entire generation.
And the two are inextricably linked. Lose the reform referendum and the financial markets will doubt the Government's ability to deliver the single currency for Britain. The euro needs a sustained cross-party approach to Britain's European policy; PR provides the framework for British politics that makes EMU a credible option for sterling.
Paradoxically, it is the referendum that is planned to take place later, on the euro, that has attracted most attention and where preparations are most advanced. By contrast, preparations for the referendum on voting reform are hardly out of the starting blocks, even though the decision could be on us as early as next year.
Reformers in all of the political parties have been strangely quiet ever since Roy Jenkins and his commission began their work, with the result that few people are even aware that the issue is "live". Worse still, much of the political and communications initiative is already leaking away to opponents of reform.
The Government's record is lamentable. While constitutional reform has provided New Labour's most conspicuous political achievements since the election, ministers have failed to present their reform package as a coherent whole. There has been no White Paper setting out the programme in the round, no statement of the principles behind the Government's measures, and only one or two speeches from senior ministers arguing their case.
Electoral reform should have been at the heart of this constitutional programme. Indeed, it is at the heart of the plans now being implemented for the creation of a Scottish Parliament, a Welsh Assembly and the new Assembly in Northern Ireland - all of which could have foundered without proportional voting systems.
Even John Prescott, normally seen as an opponent of reform, has proposed the supplementary vote system for the election of the new London mayor, and proportional representation for the Greater London assembly.
Yet ministers have failed to set out the arguments that persuaded them to support proportional systems for these new bodies. The Government has even declined to explain very widely that there will be a new proportional system for next year's European elections.
This silence is compounded by the stuttering nature of the public campaign for Westminster reform. Far too many people believe that nothing can or should be done until Jenkins has reported on his commission in October and, particularly, until Tony Blair has reacted sometime thereafter.
This "waiting for Jenkins and Blair" approach sits oddly with a movement that should be about winning democratic change from the grass roots. Moreover, the ceasefire is not being observed by those who oppose change. Every week, inaccurate statements in the media go unanswered by reformers.
Winning the referendum will take hard work and imaginative campaigning to convince people of the advantages of proportionality. The arguments each way will not be clear cut. The polls show that despite the general election, public dissatisfaction with politics and politicians remains huge. This referendum will be as much a battle between hope and fear as any general election campaign.
Equally, the campaign will be a battle to win votes for the new politics that flow from electoral reform. Reformers will only persuade people to vote for this change if they win acceptance of its consequences.
To a limited extent this is happening. Tony Blair and Paddy Ashdown have established a constructive relationship that enables them to co-operate across party boundaries, while also maintaining their distance on issues where they disagree.
This approach represents a more honest form of politics. It helps to persuade voters that their politicians are doing something to "clean up the system", as they said that they would.
The reform case needs to see more of this style of politics - between parties as well as their leaders. As it succeeds, voters will appreciate that there is something better on the other side of a reform referendum.
Prepared properly, coalitions can be far more open, transparent and democratic than the sort of single-party, top-down politics we are used to - especially when the governing party is as tightly controlled as New Labour. They bring new and better ways of holding politicians to account, and for forming high-quality public policy. Coalition politics is based on public agreements; Westminster politics is based on private deals.
This argument should be on the political agenda now, before Jenkins reports and during this autumn's conference season. His proposals will make sense only if it is widely understood that reform at Westminster makes coalitions likely.
The ground should be properly prepared. This requires Labour to admit that it does not have solutions to all the country's problems. And Liberal Democrats should do more to demonstrate that they are participants in a movement of reform that reaches wider than their party.
Alan Leaman and Andrew Duff are members of the Liberal Democrat policy committee