Nick Clegg has appealed to voters in Eastleigh not to exact retribution on the Liberal Democrats for Chris Huhne's conduct.
Lib Dem chief whip Alistair Carmichael will move the writ on Thursday, formally triggering the by-election in the Hampshire constituency following Huhne's decision to quit in the wake of his conviction for dodging speeding points.
Mr Clegg said that the former cabinet minister would at some stage have to explain himself to his erstwhile constituents. But speaking at the Institute for Government in London, the Lib Dem leader urged people not to take revenge on his party.
"What has happened has happened with Chris Huhne and his guilty plea on Monday morning," he said.
"I think the choice for the people of Eastleigh in the by-election is what kind of an MP do they want to have now. I hope that it will be on that basis rather than in a mood or spirit of retribution that the debate will be conducted in Eastleigh."
The Lib Dems, as the sitting party, have opted for a short, three-week campaign with polling to take place on February 28. They face a bruising battle with the Conservatives, their coalition partners, who finished second in the 2010 general election.
Tory Party chairman Grant Shapps has already said that the people of Eastleigh had been "sold a lie" by Huhne.
Mr Clegg, however, insisted that he had for many years been a good constituency MP.
"Whatever the rights and wrongs of the court case, I think you will find that if you go to Eastleigh everyone recognises that he was an extremely hard-working local MP," he said.
"I am not going to provide a guesswork commentary on how the Conservatives want to fight the by-election. We will fight it, in part, on our record of commitment to serving the people of Eastleigh well. Chris was a very good constituency MP, whatever other things happened elsewhere in his life."