The Prime Minister ranked defeating Labour in 2015 above maintaining the Union in a speech to the National Conservative Convention in London.
Despite previously saying he would fight with "every fibre of my being" to keep Scotland in the United Kingdom, Cameron said "the biggest fight of all" for the Tories was the election.
He said his party faced "some huge fights on the horizon" - European and local elections on May 22, the referendum on September 18, and the "almighty fight to keep Labour out" of government at Westminster on May 7, 2015.
He said in all three contests the Tories had to be clear and unapologetic about what they stood for, and bold when taking on opponents.
For the European elections, that meant telling voters the party stood for the British national interest and that Ukip "can't change anything".
In the section of his speech headed "UK referendum" rather than Scottish referendum, the Prime Minister said the party needed to be clear about its love of the UK, and the bonds created over the past 307 years.
The "ties that run rich and deep" included the industrial revolution, the sacrifices of the First and Second World Wars and the NHS, he said, and he was proud, not apologetic, about Britain's history.
He said: "This is what I love about the UK. The decency. The family. The solidarity. The health service we built together, the help in hard times we provide together, the starving people around the world that we help feed together, the freedom we fought for together.
"Our incomparable United Kingdom - we love this country and all it stands for - so let's win this referendum for all our futures."
The emphasis on the UK as a family of nations was part of the No camp's recent switch to a more positive, emotional appeal to voters, known as "love-bombing".
But Cameron also said the Conservatives had to be bold in challenging nationalist arguments.
He said: "Frankly, Alex Salmond is all over the place … The reality is he's a man without a plan and let's be bold in saying it."
Then, referring to the 2015 General Election, he added: "Just over a year from now, we face the biggest fight of all.
"We talk about 'high stakes elections', but the stakes in this one are truly stratospheric."
He said a win for a "monumentally weak" Labour Party would send Britain backwards, reversing recent economic progress, costing jobs, putting up taxes, and adding to the national debt.
MP Angus Robertson, the SNP's referendum campaign director, said: "People across Scotland will no doubt be surprised to hear David Cameron describe the 2015 General Election as 'the biggest fight of all'.
"With less than six months to go until the referendum, he is not even trying to hide the fact his biggest priority is a Westminster election that is over a year away.
"The Prime Minister should be more concerned that the No campaign is losing credibility by the day. David Cameron is not serious about Scotland - and it shows."
Deputy First Minister Nicola Sturgeon said if Cameron felt secure about his arguments he would debate on TV with Alex Salmond instead of delivering "yet another lecture from London."
She said: "Mr Cameron talks about the importance of the health service as he steadily dismantles it south of the Border, and he talks about providing help in hard times as his Government's welfare cuts force more and more people in the UK to turn to food banks."
Sturgeon also said yesterday the referendum was a "now or never" moment to remove nuclear weapons from Scotland, with a Yes vote meaning an end to Trident at Faslane.
Speaking at a CND Scotland rally in Glasgow, she said: "Just think about it - as the world's newest country, one of the first things an independent Scotland will have the chance to do is rid itself of weapons of mass destruction.
"I cannot think of any more powerful statement we can make to the world about what kind of country we will be."