Conservative MPs are already talking up their party's chances in the Eastleigh by-election triggered by Chris Huhne's resignation , with one, Alec Shelbrooke, describing  it as "an early opportunity to exact revenge on the Lib Dems over boundaries". The Tories have no intention of going easy on their coalition partner; this is a must-win seat for them.
Given how poorly the Lib Dems are polling nationally and the slimness of their majority (3,864), it's unsurprising that many expect a Tory victory. But Lib Dem activists are confident that they can hang on. They point out that the party has actually gained seats in recent local elections, increasing its majority  on Eastleigh Borough Council from 34 seats to 36 in May 2012 (the Lib Dems hold 40 to the Tories' four). The Lib Dems, who plan to treat the next general election as 57 by-elections, have long argued that they will lose fewer seats than expected in 2015 because their vote is holding up in key local strongholds. The by-election will be an early test of this claim.
It is no less of a test for the Tories, whose hopes of winning a majority in 2015 depend on them taking a large number of seats off the Lib Dems. The party has included 20 Lib Dem MPs  on its 2015 target list of 40 in the belief that they will prove easier to dislodge than their Labour counterparts. Were the list purely based on the swing required, only nine would appear. But if the Tories fail to win Eastleigh, even after the sitting MP has been forced to resign in disgrace, a Conservative majority in 2015 will begin to look impossible.