When questioned during the live TV debates before the General Election about cuts to tax credits, David Cameron told the British people he wasn't going to even touch them. As a result, the Conservatives no doubt won the votes of many people whose tax credits are soon to be cut.
And we all know now that those votes were won on an outright lie.
That is why today in Parliament Labour will call on the Prime Minister to stick by what he promised the British people only a matter of months ago.
Because it is this kind of politics that the public can't stand. It is the same old politics that saw the Lib Dems dumped by the electorate in May.
In contrast, last month at the Labour Party conference in Brighton, Jeremy Corbyn and I set out our vision for a new way of doing politics - straightforward and honest.
We will not tolerate a Government trying to mislead people and we will call it out when it does. That is what the Prime Minister tried to do when he told the House of Commons in September that the Summer Budget changes will mean that families losing tax credits will be "£2,400 better off."
Because not very long afterwards, analysis by the House of Commons Library revealed that in fact these three million families, working on low wages, will lose around £1,300 a year.
It is clear the cuts to tax credits amount to a work penalty for middle and low income families across the country. This is underlined by the independent Institute for Fiscal Studies who have said that it is "arithmetically impossible" for the Government's so called 'National Living Wage' to make up for these losses.
It was in light of this that Labour's Shadow Chief Secretary to the Treasury, Seema Malhotra MP, wrote to the Prime Minister earlier this month, asking how the Government's figures were calculated; and why they differ so starkly from those calculated by independent bodies like the IFS.
She is yet to receive a response.
Therefore, faced with a government that will not be straight with the British people before or after an election, Labour will act. Today Labour will pledge that when we return to power, we will reverse these changes, making sure these families are not losing out.
This is not just because it is the right thing to do for those affected; and it is not just because we believe the tax system should not penalise people for working - it is also because we believe that politicians should not abuse the trust of voters given to them during an election campaign.
Of course we want to see a Britain where fewer people rely on tax credits to make ends meet, but we want to see this happen through a true high wage economy - not inadequate measures like the so-called 'National Living Wage'.
But this will be of little comfort to those receiving letters in the post this winter telling them how much worse off they will be. That's why we are saying to George Osborne, it's not too late to act. We call on the Chancellor to scrap his ideological cuts to tax credits and abolish the work penalty.
What the decision on tax credits comes down to is a question of principles and in whose interest the country is being run. The Tories say there is not enough money to reverse these changes, slashing people's incomes while still managing to pay for tax breaks to the wealthiest in our country.
Even Tory MPs like David Davis, Boris Johnson, Zac Goldsmith and many more in their own party are waking up to what we have been saying.
The Tories should learn the lessons from their their former Coalition partners.
In May, the Lib Dems were not forgiven by voters for the false promises that they gave before the 2010 election on tuition fees. And rightly so, the public want politicians who will be honest with them. The Tories need to keep their promises on tax credits or face being the new Lib Dems.
The Tories might arrogantly hope that voters will forget, but decisions like this define governments. And the three million people who have been lied to certainly won't forgive the Tories for the false promises they made on tax credits in May at the next election.
Tory MPs should ask themselves, what is more important, tax breaks for the few or keeping your promises to the many? Because we know the answer and if this government doesn't reverse these changes to tax credits, then Labour will.
John McDonnell is Shadow Chancellor and the Labour MP for Hayes and Harlington