They have good reason to worry. The demands on the service posed by an ageing population and medical advances are rising. But spending plans up to the millennium, on which both main parties agree, show virtual zero growth.
Waiting lists are rising and health authorities, short of cash, are arbitrarily restricting treatments.
The unequal provision of NHS care in different parts of the country is forcing patients to change their addresses in order to obtain the treatment that they need.
In the last week, The Independent has learned of a patient who has waited 13 months for heart surgery, of a cancer sufferer forced to rely on charity to pay for her drugs, a sick woman left for 14 hours on a trolley while doctors searched for a vacant bed and a multiple sclerosis sufferer who faces the prospect of moving to another part of the country to obtain the treatment that he needs.
Medical organisations say inequity is growing and the notion of the NHS providing equal care for all is a myth. They say rationing is inevitable and that with a fixed budget the pressure to ration will grow. The parties refuse to accept the logic of the argument, insisting that with adequate funding rationing can be avoided.
Last week, nine medical charities wrote to the three main party leaders calling on the next Government to monitor the availability of treatments on the NHS and issue central guidance on which should be provided.
Peter Cardy, chief executive of the Multiple Sclerosis Society, said: "The impact of ill health and disability is identical wherever people live but availability of treatments is all too often dictated by geography. We believe the present lottery of care is totally unacceptable. While there must be limits on NHS resources, arbitrary rationing is not the way to manage them."
Variations in provision also affect routine services. Heart surgery rates vary threefold among districts and chemotherapy for cancer varies twofold.
Ken Judge, director of the policy institute at the Kings Fund, the health policy think tank, said: "Most people think we have a National Health Service, but what we actually have is 200 local health services. If resources are fixed and cost pressures increase, it is even more important to distribute the available resources fairly. There must be more honesty and explicitness over what the NHS will provide."
Some analysts argue that Britain needs a "Bill of health rights", setting out what the public may expect from a publicly funded health service.
This ought to be preceded by a public debate on which elements should be considered part of the "core" service and which are optional extras.
The argument between the two main parties has focused not on these substantive issues but on the cost of running the NHS internal market, put by Labour at pounds 1.5bn a year.
The Tories are sensitive to the charge and last year Stephen Dorrell, the Secretary of State for Health, ordered a 5-per-cent cut in management costs. Labour pledges to save pounds 100m in the first year by reducing costs in the most over-managed authorities and trusts. Over the long term, Labour claims it can cut bureaucracy by 90 per cent. It says these savings will fund long-term growth in services.
Mr and Mrs Butler plan to vote Labour, to save the NHS. But the service needs a real increase of 3 per cent a year in funds, equivalent to pounds 1bn, to meet rising demands. NHS managers say it is inconceivable that management savings will yield that sort of sum.
The future of the NHS rests on the tax-and-spending plans that neither Labour nor the Tories have the honesty to disclose prior to the election. That leaves voters, such as the Butlers, making a leap in the dark.
Change since last week
Labour 48% -1
Con 30% -1
Lib Dem 15% +2
Conservative predictions that Wednesday's ICM poll heralded a "collapse" in Labour's poll ratings are scotched by today's Independent/Harris poll, showing Labour's lead unchanged at 18 points.
ICM's dramatic poll for the Guardian showed Tony Blair's lead cut from 14 points to five in the space of a week, but now three other polling companies which were carrying out interviews at the same time report no significant change.
MORI in the Times yesterday put Labour's lead at 21 points, up four from the previous week, while Gallup in the Telegraph has Labour's lead unchanged at 20 points.
While most pollsters remain nervous about their predictions of the levels of support for the parties, it seems there has been no significant movement in public opinion yet - on the surface at least.
Harris Research interviewed 1,177 adults face-to-face in their homes between 18 and 21 April.Reuse content