Liberal Democrat Cabinet minister Danny Alexander has warned his Conservative coalition partners they have a "responsibility" to support House of Lords reform.
Amid fresh tensions over the plans, which are expected to be unveiled tomorrow, the Chief Secretary to the Treasury insisted both parties had a duty to implement the Coalition Agreement.
In a speech to the Electoral Reform Society Conference, he said both the Tories and Lib Dems would be judged on their ability to deliver on their promises.
Although Prime Minister David Cameron backs Lords reform, many in his party are critical of the changes and the attention that is being given to them when the economy is still on the rocks.
More than 100 Tory MPs are said to be ready to rebel over the reforms.
Mr Alexander will say that finding a way back to economic prosperity is his "number one priority" but add that it is not "the limit of this government's ambitions".
He will say that Lords reform was one of the many "ambitious" reforms included in the Coalition Agreement.
"In all of these areas and many others, both parties in this coalition continue to have a responsibility to deliver - right to the end of this parliament," he will say.
"Because, even for those that don't like to admit it, both partners in this coalition will be judged, not on the ambition of our rhetoric, but on how effectively we can continue to work together to implement the policies we jointly set out in the Coalition Agreement. All of them."
He will dismiss critics who suggest "the whole of government can focus on delivering on only one thing at a time" and urge Labour to also back Lords reform.
"Right across the political spectrum, we all have a responsibility to deliver reform to the House of Lords, because it is what we promised
"Liberal Democrats, Conservatives, the coalition government, and yes, the Labour Party too.
"We all collectively should feel the pressure to deliver, because it is what all political parties put in their manifestos.
"And we should see that commitment through, not because it is the number one thing that comes up on the doorstep, but because it is the right thing to do, because in a democracy law makers should be elected."