David Cameron: Coalition faces 'choppy period' over AV
David Cameron has told the BBC it was obvious the coalition would face a "choppy period" over the referendum on changing the UK voting system.
He also admitted he had had to play a bigger role in the "no to AV" camp than he had initially intended.
It follows accusations of "mudslinging" during the campaign, in which the Lib Dems and Tories are on opposite sides.
Earlier Deputy PM and Lib Dem leader Nick Clegg criticised the "dinosaurs" who oppose a switch to AV.
UK voters will be asked on 5 May whether they want to change the voting system for electing Westminster MPs from first-past-the-post to the alternative vote, where candidates are ranked in order of preference.
As the campaign enters its final two weeks, there have been accusations of "mudslinging" and "appallingly personal" attacks on Mr Clegg by the no campaign.
Lib Dem cabinet minister Chris Huhne said he had been "shocked that coalition partners can stoop to a level of campaign that we have not seen in this country before".
In an interview with BBC political editor Nick Robinson, Mr Cameron said: "This is obviously for the coalition going to be quite a choppy period because we are embarked on a referendum where we are on different sides.
"We always knew this would be the case and what matters is that we both make our arguments... and explain to people the arguments. I'm passionate that no is the right answer, Nick is equally passionate that yes is."
"We just must have that argument and make sure once it's over the coalition is and remains strong and I believe that's the case."
He said he was not in control of the main "no" campaign - which has been accused of personal attacks on Mr Clegg - and said the Conservative campaign had been "thoroughly reasonable".
Nick Robinson said some Lib Dems had complained privately that Mr Cameron had said he would keep a low profile during the AV campaign and had let Mr Clegg down.
The prime minister said: "Nick and I had both discussed at the time that we were going to fight this referendum vigorously.. I think frankly both campaigns have ended up with more politicians in than perhaps either of us had hoped, we hoped there would be other figures coming forward and that hasn't happened."
Coalitions 'the norm'
He said he was "taking quite a high profile in the campaign" but a lot of people were not "really engaging in this argument" and he and Mr Clegg could play a role in bringing the arguments to life.
But he said the coalition was "strong", the Lib Dems had made an important contribution and the supporters of both parties could be pleased.
He denied that he was referring to Mr Clegg in a speech on Monday, when he said AV would damage the "chain of accountability" between pledges politicians put in their manifestos and what they deliver in government.
But he argued that av would mean coalitions would be "the norm" rather than the exception and while it was "good for Britain" at the moment, he said: "I don't believe coalition is always good for Britain."
In a speech earlier Mr Clegg defended the work of the coalition government and said, whatever the result of the referendum, "the work of this government will continue in the national interest".
He said he had been unable to deliver "the whole Lib Dem manifesto" but said: "I will continue to make what are sometimes difficult compromises but ones which are always shaped as best I can by the Liberal values I hold dear."
Mr Clegg added: "There are people on the left and the right who preach new politics and pluralism and yet are now so damning of its inevitable consequence, which is compromise.
"You can't claim to stand for a new kind of politics, for a new kind of pluralism, and then vilify those who try to practice it."
In his speech Mr Clegg criticised "paternalists and conservatives" lining up to try to block AV and accused "dinosaurs" on both the left and right of "a desperate attempt to defend the indefensible".
The Labour Party is split over AV - leader Ed Miliband supports changing the voting system but many senior politicians in his party have joined the "no" campaign.