The Prime Minister told Tory party activists that he wanted to push his “radical” agenda as he warned that the country faced a “truly dreadful” future if Jeremy Corbyn led Labour into power.
Mr Cameron used his speech at the Conservative spring forum to call for discipline - but splits over Europe were evident as pro-Brexit Boris Johnson looked forward to an “independence day” after the June 23 referendum.
The Prime Minister, who acknowledged he could have better handled the row over his shares in the offshore trust set up by his father, attempted to rally the Conservative troops ahead of May’s elections.
The conference in central London took place amid tight security as protesters gathered outside demanding Mr Cameron’s resignation.
The Tory leader told activists: “We have a huge responsibility over the coming months and coming years to settle this issue over Europe - and it’s right we are having that referendum - to show that discipline and unity of purpose that is vital for government.”
The Prime Minister used his speech to joke: “Let me tell you one thing I’m not going to talk about today - and I think there will be widespread relief - and that is Europe.
“Boris and I agreed we weren’t going to talk about Europe.”
But as audience members indicated Mr Johnson had indeed mentioned the EU, Mr Cameron said “Whatever side we are on, let’s not forget why we are having this referendum.
“We are having it because this party, this Government, made a promise to the British people, is keeping a promise to the British people so after 40 years they get their say.”
Mr Johnson could not resist a jibe at the European Union ahead of the referendum.
Activists arriving at the event were met by campaigners on both sides of the EU argument in a demonstration of the divisions within the Tory ranks.
The Brexit-backing Mayor told activists that Brussels is “a city I love though not necessarily all the institutions therein”.
London has 400,000 French citizens living in it “which will remain the case” after June 24 - or “independence day” as Mr Johnson dubbed it.
He said the latest James Bond movie, which featured his City Hall office in a climactic scene, was a “very important study in what happens to an unelected bureaucratic cabal”.
Remain supporter Home Secretary Theresa May told the event: “The referendum is a question to which the answer will define our country for years to come.
“But we mustn’t allow it to define our Party. Being a Conservative is more than about being Leave or Remain.
“It’s about making sure Britain is the best possible country for our children to grow up in and for our elderly to grow old in - the best possible place for our families, friends and neighbours to live and thrive and prosper.”
In an attempt to restore the Tories’ “compassionate Conservative” brand following the resignation of Iain Duncan Smith, the Prime Minister used his speech to insist that the “modern” party was committed to opening up opportunity for all.
He told the event in central London: “People who have got a name that doesn’t sound that English don’t even get the call-up for the interview.
“People still get discriminated against because of the colour of their skin, because of the person they choose to love, sometimes because of the sex they are - a woman rather than a man.
“If we are going to be the true party of opportunity - and that should be our goal - if we are going to break down all the barriers to success in our country, we have got to be every bit as passionate about fighting discrimination, fighting racism, fighting disadvantage in all corners and pockets of our country as we are about creating jobs and great schools.
“That is the modern Conservative party.”