The Prime Minister dismissed the level of influence Nick Clegg's party had over last week's Autumn Statement, insisting the measures unveiled had been "distinctly" Conservative.
Lib Dems would "prop up a failing Labour government" after the general election next year putting the "hopes, dreams, and livelihoods" of millions of Britons at stake, he claimed.
Since the Autumn Statement both parties have publicly distanced themselves from each other.
The Prime Minister's brutal intervention comes after George Osborne claimed the Lib Dems would wreak "economic chaos" if they were left to pursue their own agenda.
Clegg hit back, accusing the Tories of "kidding" voters over the scale of cuts they would impose if they win next May.
In an email to Conservative MPs, Cameron wrote: "At the end of an important week for the UK economy, the British people face a clear and important choice: between competence and chaos.
"All Labour offer is more of the same: more spending, more borrowing, more taxes, more debt. They would threaten the financial security of families across Britain.
"The Liberal Democrats are all over the place, unable to decide whether they want to stick to the plan or veer off it. And they - like Ukip - would be prepared to prop up a failing Labour Government.
"In contrast, the Conservatives offer a long-term economic plan that is working. As the Autumn Statement confirmed, the deficit is falling, unemployment is falling, and our economy is growing. Indeed - ours is the fastest growing of any major advanced economy in the world.
"It was also a distinctively Conservative autumn statement. By overhauling stamp duty, supporting business, and letting husbands and wives inherit their partner's Isa tax-free, we are backing people's deeply-held aspirations to better their lives, to own their own home, to work and to save."
The Deputy Prime Minister was accused of trying to distance himself from the autumn statement after failing to show up for the set-piece Commons event, but has insisted he fully supported the measures that had been announced.
But he said his coalition partners must "come clean" about their future plans to wipe out the deficit.
"I just think the Conservatives are kidding themselves and seeking to kid British voters if they are claiming that it is possible to balance the books, deliver unfunded tax cuts, shrink the state and support public services in the way that everybody wants," he told BBC One's Andrew Marr Show.
"It just doesn't add up."
He warned, however, that a future Conservative government would "remorselessly" pare back the state every year, hitting public services and the working age poor "very hard".
Clegg added: "I do think the Conservatives now need to come clean because they are not being straightforward with people. How many colleges will be closed, how many primary schools will be closed, how many police will be taken off our streets, how many people who are in the working age poor will be hit by these plans?"
Danny Alexander waded into the row to accuse the Tories of "pandering to Ukip" in a pre-election "panic".
The Chief Secretary to the Treasury claimed the Tories would "inflict unnecessary pain" on the country because they were "ideologically committed" to shrinking the state and had a policy of "austerity forever".
In an article for The Daily Telegraph, he wrote: "It is sad to see the Conservatives move away from the sensible, balanced approach of the coalition, to a more doctrinaire policy that would inflict unnecessary pain on the people of Britain.
"Who'd have thought that of the two parties that formed the coalition, it would be the Tories who would be blown off course?
"A mix of unfunded tax promises, harsh spending plans and pandering to Ukip may be born of pre-election panic, but it is not economically credible."
Alexander said it was "totally unsurprising" that the two parties of the coalition should seek to highlight to voters that they remained distinct but declined to rule out sharing power again with the Tories in the event of another hung parliament.
Asked if it was possible for the present administration to survive increased skirmishes between the partners, he told BBC Radio 4's Today: "Absolutely it is.
"What we have done, and we showed this last week in the Autumn Statement, is work effectively together to deliver a very Liberal Democrat package of measures with lots of income tax cuts, tax reforms, sticking to the path on the public finances.
"So we have shown that we can work well together in this Parliament. What we are doing is something that should be totally unsurprising, which is two political parties - with very different ideologies - setting out their views about the future of this country in a clear and distinct way and I am going to continue doing that for the next five months and beyond but that does not in any way undermine our ability to work effectively together in this coalition to keep the country on the right path."
He said: "We would cut less than the Tories, but borrow less than Labour.
"That way you can have both a strong economy and a fair society. We are the only party that can deliver both of those things, unlike the other two who are increasingly lurching away from the path we have been on in this Parliament."
Pressed on whether there was any prospect of another Conservative/Lib Dem administration despite being "set out on very different paths", he said: "As we said at the general election in 2010, we would seek to talk first with whichever party had the strongest mandate in the event of a hung parliament.
"That is the responsible thing to do.
"What we are doing now is setting out precisely and clearly what are the Liberal Democrat policy objectives in the next parliament."