The Tories would be the odds-on favourites to hang onto the Newark seat if Tory MP Patrick Mercer quits before the next election.
Mercer is under pressure to stand down as an MP after he quit the Tory party following a sting by the BBC's Panorama.
He has signalled his intention to remain as an independent member until the next General Election, but critics have said he should go now.
Bookmakers did not waste any time in issuing odds should a by-election - which David Cameron will be desperate to avoid - be called.
Paddy Power said the Conservatives would be 1/3 favourites - with Nigel Farage's Ukip second at 4/1. Labour are 5/1 shots, with the Lib Dem decline highlighted by their remote 90/1 odds.
At the 2010 General Election, Mercer had a handsome 16,152 majority. Labour just pipped the Lib Dems to second, while Ukip were a distant fourth with just 1,954 votes.
Ukip leader Nigel Farage has already said he will not stand in any by-elections, promising to pick a seat for the 2014 General Election.
But Ukip pushed the Tories into third place in the recent Eastleigh by-election, while Farage's party also bagged more than 130 seats in the local elections.