High-profile peers including the former leader of the Liberal party, David Steel, have written to their MP colleagues urging them to vote against plans to elect the upper chamber.

Former chancellors Lord Howe and Lord Lamont and former Speaker of the Commons Lady Boothroyd are among 40 peers who have written to MPs to urge them not to back a reform that would "politicise" the House of Lords.

Nick Clegg will present the plans to the Commons on Tuesday for an 80% elected upper house but in a marked departure Conservative politicians will take over the presentation of the issue after that point with the Tory minister for political and constitutional reform, Mark Harper, and the Tory leader of the Lords, Lord Strathclyde, leading the media push. The deputy prime minister will propose a shopping list of options. The draft bill will include options for an 80% or 100% elected chamber of between 300-350 members.

A joint committee of MPs and Lords will consider the options for up to a year before parliament will be asked to vote.

The Lib Dems are distancing themselves from the perception they are overly invested in Lords reform after a bruising performance in the alternative vote referendum when so much of the country voted against the move to change the system. They are concerned that they will be too heavily associated with another piece of constitutional reform when people are experiencing a squeeze on living standards.

A source said: "We're not just constitutional reform obsessives, House of Lords reform is not the be all and end all for us. We've got big gains on health, education, civil liberties. We've rebalanced the economy, brought in a green investment bank, reformed the tax system, brought in a windfall tax on oil and gas and more. We're not just about getting through House of Lords reform."

They are also keen the public be reminded that reforming the House of Lords is as much a Tory policy as a Lib Dem one, and indeed was in every party's manifesto.

The draft bill proposing an 80% chamber will be launched alongside a white paper which will include a range of options including a 100% elected house. It is not being rushed through but instead it will have an entire year of pre-legislative scrutiny in a joint committee of both peers and MPs from all parties, before it goes through. "We have learnt that we have got to get buy in. So we are giving people options and are taking a consensual approach," the source said.

"The key lesson to learn is that we mustn't let the best be the enemy of the good."

They are also seeking to put the spotlight on the Labour party, who they believe did not fulsomely support them in the run up to the AV referendum because it was divided on the issue, with oppositions to the pro-reform position of Ed Miliband coming from the House of Lords in particular.

The Lib Dem source said: "This is another test, directly for Ed Miliband. Does Ed Miliband have the leadership to push this through the party?"

In their letter to MPs, the peers say: "We believe that supremacy and authority would inevitably be challenged if the House of Lords were to replaced by a wholly, or largely, elected second chamber. Not only that. Each individual member of parliament would be in the position of having an elected 'senator' able to claim jurisdiction within his or her constituency.

"And if that 'senator' were elected by proportional representation, and for one single 15 or 20 year term, he/she would certainly seek to claim greater legitimacy, and, being ineligible for re-election, would have no accountability to the electorate."

"We are not saying that the House of Lords should not be reformed. It is too big and there are measures that could be taken to make it even more effective than it is at the moment.

"We believe, however, that an appointed house does have very real merit. It can deliberately reflect the diversity of the nation in a way that party selection and elections can not."

They add: "The inevitable consequence of elections would be a greater politicisation of the House of Lords and a reduction in the independent element within it. Please bear in mind that if 20% of the House of Lords were appointed, to preserve something of that independent element there would then be two classes of members, the elected and unelected."