Asked earlier on Friday why he had acted against the wishes of Mr Cameron, Mr Davis said: "We're a party not a regiment and we're in politics hopefully, and virtually in all cases, for reasons of principle."
He said, despite the risk, it was not a "moment of madness" but a decision taken after a week in which "fundamental liberties, habeas corpus" had been "eroded almost by a process of bullying and bribery in the House of Commons".
"Frankly I thought we had got to the point where somebody had to make a stand," he told BBC Radio 4's Today programme.
Later he told BBC Radio 5 Live: "This is an issue that is more important to me, at the end of the day, than my job if I lose."
Mr Davis attacked the shabby and squalid process" he says Gordon Brown used to win Wednesday's vote on 42-day detention - a piece of legislation he described as "frankly evil".
And he said if Labour refused to take him on in what would effectively be a mini-referendum on its anti-terror legislation, "they're going to show that they're ashamed of their own policies".
The BBC has been inundated with calls, texts, e-mails and blog comments praising David Davis's decision yesterday and some have questioned why I have suggested it may be a nightmare for the Conservative Party
Friday's front pages are dominated by claims Mr Davis's move has left the Conservatives in "turmoil" and handed a "gift" to the struggling Labour Party.
But Mr Davis denied he had shown contempt for Mr Cameron and had told him about his plans the night before he made the announcement - he said the Conservative leader had expressed concerns it was "very risky", but said he would bear the greatest risk.
Mr Davis said he had received a "massive response" from the public, including some Liberal Democrat and Labour activists.
He pointed to the BBC website's own Have Your Say page as evidence of support outside the "Westminster Village" and said he had been flooded with offers of help and cash from supporters.
A poll for Tory grassroots website Conservative Home suggested 65% of party members were "inspired" by his move.
But Mr Brown is convinced the public are with him on 42 days - and he seized on Mr Davis's resignation as a sign of "deep divisions" in the Tory Party.
Speaking after a breakfast meeting with UN Secretary General Ban Ki-moon, Mr Brown said: "Everyone now recognises this is a stunt that has become a farce."
He added: "At the first test of what their policy is on the big central issue of national security, the Conservative Party are totally divided."
David Cameron has said he would campaign in the by-election but stressed it would be a "personal campaign by Mr Davis.
"Many people will admire him for taking that stand but it is a personal decision, it is not a shadow cabinet decision, it is very much his campaign," said Mr Cameron earlier on Friday.
The proposal to extend maximum detention limits passed through the Commons this week by a margin of nine votes, with 36 Labour MPs joining the Tories and Lib Dems to vote against it.
But the government faces a battle in the Lords to get on to the statute books.
Shadow attorney general Dominic Grieve has been appointed as the new shadow home secretary.
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