Lib Dem leader Nick Clegg says the Tories should stop "hammering" David Davis over his surprise resignation.
Several Tory MPs have criticised his resignation as an MP and shadow home secretary, in protest at the extension of pre-charge terror detention limits.
But Mr Clegg said it was "pretty rich" to brief against the architect of Tory policy on civil liberties issues.
Mr Davis said Gordon Brown should put up a Labour candidate against him, and accused the PM of "gutlessness".
Meanwhile Labour rebel Bob Marshall-Andrews said he would support Mr Davis - and said his party should join the contest.
Mr Davis stepped down on Thursday - the day after MPs voted by a narrow margin to extend the maximum time terrorism suspects can be held, before they are charged, from four to six weeks.
His resignation forces a by-election in which he will try to regain his seat, campaigning on civil liberties issues.
But the announcement, apparently against the wishes of party leader David Cameron, has been criticised by some Conservatives.
Former defence minister Nicholas Soames told the Sunday Times it was "foolish". Shadow defence secretary Liam Fox told the Sunday Telegraph: "It's clearly a decision he made for himself."
The way in which he is now being hammered by some really nasty briefing from his own side is pretty rich
Defending his decision not to stand a Lib Dem candidate against Mr Davis - whose Haltemprice and Howden constituency had been a top Lib Dem target - Mr Clegg, whose party also opposes the extended detention limit, said some issues "go beyond party politics".
He added that without Mr Davis the Conservatives might not have opposed extended detention limits and ID cards so strongly.
He told BBC One's The Andrew Marr Show: "The way in which he is now being hammered by some really nasty briefing from his own side is pretty rich given that he has done quite a lot of heavy lifting in the Conservative Party on these kind of issues."
Shadow foreign secretary William Hague told Sky News the Conservative Party was "united" on opposing the 42-day detention limit.
"Do we wish that he had stayed in his post as shadow home secretary? Well, yes we do - certainly I wish that very much. But I hope he is safely returned to Parliament with a good majority. I think we have taken this in our stride."
But Labour's Chief Whip Geoff Hoon told Sky News: "I think it's more to do with the row inside the senior leadership of the Conservative party than it has anything to do with a Parliamentary process."
I think he is being self-indulgent and has made a wrong decision that will backfire on the Conservative Party
Andy Burnham Culture Secretary
He said he was surprised that, with the House of Commons having voted on the issue, Mr Davis believed it "appropriate to hold a by-election in his own constituency".
Labour has not yet announced whether it will field a candidate, but Gordon Brown has already dismissed Mr Davis's challenge as a "farce".
The proposal to extend maximum detention limits passed through the Commons last week by a margin of nine votes, with 36 Labour MPs joining the Tories and Lib Dems to vote against it.
Among the Labour rebels was Mr Marshall-Andrews, who told the BBC: "I think David Davis is absolutely right to take this step, we need to have this debate and I'm very sorry my own party isn't going to field a candidate."
He said he thought other Labour MPs would join him and said civil liberties was an issue "worth fighting for, whatever your party".
Earlier Mr Davis told the BBC he believed the Lords would reject the 42-day limit but that the government would use the Parliament Act to eventually force through a piece of law he described as "a gross offence to justice".
He said it would be "gutless" if the prime minister "bottled it" and did not field a candidate to stand against him.
But even if no mainstream party stood against him, he said: "We will make the arguments anyway", saying the Iraq war veteran Col Tim Collins would be among people joining "a huge campaign".
Asked about criticism of his decision he said: "I knew it would do me harm - that's self-evident."
But he said he did not believe it would harm the Conservatives and said a Sunday Times poll suggested they had gone up two points in the polls since Wednesday.
"This is not about an issue between me and the Conservative Party - it's an issue between me and the government," he said.
But Culture Secretary Andy Burnham told BBC One's The Politics Show: "I think he is being self-indulgent and has made a wrong decision that will backfire on the Conservative Party."
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