Most Conservatives are against moving to the alternative vote, while most Lib Dems are in favour of it.
Tory Mr Cameron said the coalition would keep its focus despite recent arguments. Lib Dem Mr Clegg said it would work "in the national interest".
Labour said the referendum result would be "bad for one party or the other".
The UK-wide vote on Thursday will decide whether to switch from the current system for Westminster elections, known as first past the post, to AV, where candidates are ranked in order of preference.
The campaign has prompted some furious exchanges in recent weeks between coalition members.
In Sunday's Observer newspaper, Lib Dem Energy Secretary Chris Huhne, along with Labour's shadow business secretary John Denham, calls for a "progressive majority" to mobilise against the Conservatives.
But the prime minister warns in the Sunday Telegraph of a "real danger of exchanging an electoral system that works for one we would come to regret profoundly".'Much bigger'
The two sides in the AV argument are mounting a fresh drive for support as the campaign nears its end.Nick Clegg said there was still "all to play for" ahead of the referendum
But Mr Cameron told BBC One's Andrew Marr show: "The work of government goes on: dealing with the deficit, reforming welfare, making sure there are quality schools for all our children.
"I think what we need to do on the last week before polling day is concentrate on the merits and demerits of the system."
He added: "The truth about this coalition is it will continue to focus on sorting out the economy and our schools and doing those things. If we succeed as a coalition, we will succeed as individual parties."
Mr Clegg said of electoral change: "It's much bigger, this issue, than the coalition government....
"David Cameron and I, every day of every week, thrash out our differences. We are different leaders of different parties. We have different values and always will do.
"We have to work together in the national interest."
The coalition agreement between the Conservatives and Lib Dems allows ministers supporting AV and those opposing to campaign against each other.Continue reading the main story
At the moment MPs are elected by the first-past-the-post system, where the candidate getting the most votes in a constituency is elected.
On 5 May all registered UK voters will be able to vote Yes or No on whether to change the way MPs are elected to the Alternative Vote system.
Under the Alternative Vote system, voters rank candidates in their constituency in order of preference.
Anyone getting more than 50% of first-preference votes is elected.
If no-one gets 50% of votes, the candidate with the fewest votes is eliminated and their backers' second choices allocated to those remaining.
This process continues until one candidate has at least 50% of all votes in that round.Labour leader Ed Miliband told BBC One's Politics Show: "I have always thought that this is likely to be a five-year government. I think it is less likely after the rows of the last month.
"I think the country will be bemused, frankly, how both sides have got so hot under the collar about an issue that I do care about and I think is important, but it is not top of most people's lives at the moment."
Labour is split over electoral change, with Mr Miliband backing AV but many of his party's MPs - and some shadow cabinet colleagues - opposed.
Shadow foreign secretary Douglas Alexander told the Andrew Marr Show that Labour had been divided over the issue since the days of Keir Hardie in the early 20th Century but had been able to continue intact.
He added: "After this result we will be working together to get Labour re-elected....
"It will take more than a change in the electoral system to help Nick Clegg hang on, or indeed David Cameron.
"Labour is in a position that, whatever the result of the AV referendum, it's bad for one party or the other in the coalition."
Research by BPIX for the Mail on Sunday suggests that 51% of people are opposed to a change to AV, compared with 33% supporting it. BPIX interviewed 2,003 people online on April 27 and 28.
Jonathan Bartley, vice-chairman of the Yes to Fairer Votes campaign, said voters were "still just wanting a bit of clarity" on the issues.
He told the BBC News Channel that AV would "represent people's feelings more fully".
But Joan Ryan, deputy director of No to AV, said first past the post was a better system, as "everybody's vote is equal".
She added that it was fairer to all and "people know where they stand".
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