Deputy Prime Minister Nick Clegg, who already faces a battle to keep some of his own Liberal Democrat MPs on-side, appealed to activists in advance of the Comprehensive Spending Review.
In a letter to Lib Dem activists, he insisted that the coalition's plans reflected their party's priorities.
He said the CSR was a "thoroughly coalition product", adding: "Liberal Democrat ministers have been involved every step of the way.
"Our values and priorities are written through the review, like the message in a stick of rock."
Treasury Chief Secretary Danny Alexander and Business Secretary Vince Cable had ensured that the package met "key Liberal Democrat tests".
"Liberal Democrat ministers have fought to ensure that the burden of the challenge ahead is shared fairly," he said. "On child benefit, capital gains and tax evasion and avoidance - this government is making the well-off pay their share.
"And for those services that matter most to the vulnerable in our society, such as health and social care and early years education, spending is being protected."
But Labour leader Ed Miliband accused the coalition Government of taking an "irresponsible gamble" with jobs and the economy.
"What the Government should be doing is putting in place a plan to reduce the deficit but also to protect growth and jobs in our country," he said as he left his London home this morning.
"What I fear we are going to hear today is an irresponsible gamble with our economy and indeed many of the frontline services that people rely on in our communities.
He added: "People will be very fearful about what is being announced today - fearful for their jobs and fearful for many of the services that they rely on up and down the country."
Departments are facing budget cuts averaging 25% as part of the long-awaited Comprehensive Spending Review (CSR), with the Ministry of Justice and Department of Work and Pensions set to be among the hardest hit.
At the DWP, there is speculation that 'universal' handouts such as child benefit and winter fuel payments could be limited to save money, on top of a major crackdown on sickness and housing benefit.
Commuters are braced for sharp rises in train fares as the Department for Transport withdraws rail subsidies, and thousands of police officers could go as the Home Office trims costs.
Meanwhile, the social housing budget is likely to be more than halved to save billions of pounds.
The least affected is thought to be the Department for Education, which has secured an overall 5% reduction.
Even the NHS and Department for International Development, which are protected from the curbs, have been ordered to make sizeable efficiency savings to ensure that cash is not wasted.