Howard's backing for EU exit could sway wavering Tories
So why should we care what a Conservative leader who lost an election a decade ago has to say about the EU?
Michael Howard was never the most popular politician with the public, but many of his party's grassroots felt he shared their views when he was home secretary. His catchphrase was "prison works".
And perhaps more importantly, they still feel a debt of gratitude for his willingness to take over his party's leadership just after the Conservatives had fallen to a new nadir in the polls under Iain Duncan Smith.
So they will give him a hearing.
If a local Conservative association member is wavering over whether to vote to leave the EU, it will undoubtedly be reassuring to them that they don't have to - in David Cameron's words - "link arms with Nigel Farage".
Instead, they can now grasp the extended hand of friendship from their former leader.
Michael Howard has denounced the current leader's negotiations as a failure.
He blames the Eurocrats - not the prime minister, of course.
But in 2005 Howard proposed a much more extensive renegotiation - encompassing social and employment legislation and withdrawal from the controversial Common Fisheries Policy.
So, again, some in the party's the grassroots may well reach the conclusion that he knows what he is talking about, and that the current leadership wasn't ambitious enough in its demands.
But perhaps the greatest significance of his intervention is personal as well as political.
David Cameron worked as an adviser to Michael Howard when the latter was at the Home Office. A dozen years on, and Howard helped his protege to the party leadership.
There are plenty of tales of the Scottish referendum splitting families. The forthcoming EU referendum is also straining long-standing relationships.
It's far more cutting - and effective - if an ally breaks with a prime minister rather than an enemy.
And Michael Howard went far beyond saying his personal choice would be to leave the EU.
He also cast doubt on David Cameron's assertion that the EU would not negotiate better deal if the UK voted to go.
David Cameron has said the idea of fresh negotiations or even a second referendum after an exit vote is "for the birds" - a sentiment shared by many others.
But if Michael Howard nurtures a seed first planted by Boris Johnson that this is possible after all, it may just embolden other Conservatives to risk a vote to leave.
The former leader is stressing the need for the Conservatives internal debate to be conducted "with mutual respect".
But you don't have to use offensive language to inflict damage.
You simply have to cast doubts that your former protege knows what's he's talking about.
This is the first time the Eurosceptic Michael Howard has actually called for withdrawal from the EU and Downing Street will perhaps fear he will encourage some others to do the same.