Liberal Democrat deputy leader Simon Hughes has reached out to anxious party members ahead of next month's local elections, insisting that the coalition is a "practical business relationship" rather than a "meeting of minds".
Mr Hughes said the Lib Dems and Conservatives would be locked in government together for five years because "to do otherwise would be irresponsible".
And he admitted that it took his party "some time" after the General Election to "get our act together and make the most of the opportunities of being in government".
Mr Hughes' comments came as Business Secretary Vince Cable laid bare simmering tensions over immigration policy, warning Prime Minister David Cameron that "talk of mass immigration" risked "inflaming" extremism.
Several Lib Dem MPs have also voiced concerns over controversial plans to reform the NHS, which have now been placed on hold as ministers conduct a "listening exercise".
In a speech to party members in Bradford, Mr Hughes said: "The coalition agreement is not a love affair, or a marriage, or even a meeting of minds.
"It is a practical business relationship. It is an agreement for five years, because we need five years to deal with the deficit, to rebuild our economy and make Britain a fairer place.
"It is an agreement for actions we have started, and so we'll finish. To do otherwise would be irresponsible."
He said the Lib Dems were "proud" to be in government and "confident" that Britain would become a "more fair, more just and more successful place".
He went on: "To be honest it took us some time after May to get our act together and make the most of the opportunities of being in government.
"But now we have done that and the public are hearing more and more about the differences Liberal Democrats are making in government, and seeing more and more the differences between a coalition government with Liberal Democrats fully participating and the only realistic alternative last year, which was a minority Conservative government - which could have become a majority Conservative government at any time."
Mr Hughes said the UK was in the midst of a "national economic and financial crisis" that required urgent action.
But he promised: "Public services are by their nature publicly funded. Driving the state to bankruptcy does nothing to support them.
"The Government necessarily has had to make cuts, but it will not make any more cuts than necessary."
Mr Hughes, MP for Bermondsey and Old Southwark, issued a rallying call to party members ahead of local elections on May 5.
He predicted that Lib Dems would "hold many seats and win many seats" because people knew the value of having local representatives who "fight for local services rather than cut local services".
He also urged people to support the alternative vote (AV), in which candidates are ranked in order of preference, in the referendum to change the Westminster voting system.
"I do not believe that it is an accident that we have seen Labour MPs in Barnsley Central and Bury and Scunthorpe commit criminal acts when they thought they had jobs for life," he said.
"Nor was it an accident that at the last election we saw many Conservative MPs stand down after committing some of the most scandalous abuses of their parliamentary expenses from safe Conservative seats.
"AV is a simple change which will make a big difference. It will allow people to express a preference, vote for the MP who they want and not who they think has the best chance of winning, and it will mean that all MPs are elected with the majority of the support of their voting constituents."