In four days, Britain votes in a referendum that is critical to our democracy and our future.
Normally, when we vote, those votes have a use-by-date. We elect Councillors, Mayors, MPs and governments for four or five years.
But the referendum on AV is about voting in a change that is permanent.
Unless enough people turn out to vote on Thursday, Britain is in real danger of exchanging an electoral system that works for one we would come to regret profoundly.
To me there are four important reasons to save the First-Past-the-Post system we use today.
The first is its simplicity. It's so simple it can be summed up in one sentence: the candidate who gets the most votes wins.
Just compare that to AV: a confusing mess of preferences, probabilities and permutations.
Leaving aside the clear danger that this complexity could encourage negative campaigning - as in Australia, where voters are greeted at polling stations by party apparatchiks with 'How to Vote' cards, telling people the exact order in which to rank each candidate - it would also throw up some patently unfair results.
Under AV, the person who comes third in people's first preferences can end up coming first in the race.
It makes winners of losers and losers of winners.
The result could be a Parliament full of second-choices who no one really wanted but didn't really object to either.
The second major strength of First-Past-the-Post is its effectiveness.
Throughout history, it has risen to the demands of the time, often with a brutal decisiveness.
That's what happened when it brought in the Thatcher government in 1979.
The British people recognised it was time for change - and the electoral system didn't let them down.
On other occasions, when the public has felt that none of the major parties have all of the answers, it has led to a hung Parliament - as it did last year.
Under AV, such decisiveness is much less likely. It will make hung Parliaments more commonplace and make it more difficult to kick out tired governments.
Indeed, if it had been in place at the election last year, Gordon Brown could still be Prime Minister today.
I can't imagine anything much worse than a voting system that leaves half-dead governments living on life support.
The third reason to save First-Past-the-Post is its efficiency.
Everyone knows this country needs to cut spending and get back to living within its means.
At this time, we need to protect those things that provide our country with real value for money.
Our current voting system does that - it's cheap to administer and comes with little bureaucracy.
There is a real danger that AV could come with additional costs, from public information campaigns explaining the complexities of AV to the extra expense of counting votes at election time.
At this time I think our money is better spent on public services than on our political system.
The fourth reason to save First-Past-the-Post is to do with our history.
Each democracy in the world has its own story, shaped by its own chain of events.
The American system, with its strong checks and balances, was born of revolution - designed to avoid the possibility of over-mighty government.
In Europe, both after the Second World War and the fall of Communism, many countries adopted other more plural voting systems, again constructed to avoid the experience of being dominated by over-mighty governments.
Britain's democracy has its own story. Two centuries ago, voting was limited to a privileged few.
Generations of campaigners fought and died to change that. Their struggle gave us the principle that sits at the heart of our democracy today: we are all equal, therefore we all have an equal say at the polls. One person, one vote.
So First-Past-the-Post isn't just one way of counting votes; it is an expression of our fairness as a country.
It is enshrined in our constitution and integral to our history - and AV flies in the face of all that because it destroys one person, one vote.
If you vote for a mainstream candidate who comes top in the first round, your other preferences will never be counted.
But if you vote for a fringe candidate who gets knocked out early, your other votes will be counted.
That means the second, third, even fourth votes of someone who supports the Monster Raving Looney Party can count as much as the first vote of someone who supports a mainstream party. That is unfair and undemocratic.
Don't take all this from me. You can judge the relative merits of First-Past-the-Post and AV by how popular they are overseas.
Our current system is one of Britain's most successful exports - used by almost half the electors on the planet, embraced and understood by 2.4 billion people from India to America.
So in the next few days ask yourself a few questions: do you want to switch to a voting system that is hopelessly unclear, unfair and indecisive?
Do you want elections that are - as Churchill put it - "determined by the most worthless votes given for the most worthless candidates"?
And do you want to rip up a valuable part of our constitution and centuries of British history for a system that is unpopular the world over? If the answer is no, make sure you get out to the polling station on 5th May - and vote no to AV.