Since the formation of the Liberal Democrats in 1988, the party has never lost a seat at a by-election.
Before 1988 the Liberals also had a record of holding seats in all by-elections since 1934 - with just one exception - and this was an exceptional by-election.
In 1957, after the death of Liberal MP Rhys Hopkin Morris, the Liberals had to defend his seat at Carmarthen in the aftermath of the Suez crisis and with a new inexperienced party leader, Jo Grimond. Grimond dithered over supporting the Liberal candidate’s views on Suez and undermined an already-lacklustre Liberal campaign. But, the biggest problem which the Liberals faced at that by-election was that the Labour candidate was Megan Lloyd George, daughter of former Liberal prime minster David and herself a former Liberal MP and deputy leader of the Liberal Party. Megan had defected to the Labour Party in 1955. Megan Lloyd George won the seat. (Somewhat aggrieved, but always respectful of titles, Liberals sang the song 'Lady Megan is a Traitor' at party conferences.)
Although the defeat at Carmarthen reduced Grimond's party to its lowest total of just five MPs, the Liberals managed to bring their representation back to six MPs by capturing Torrington the following year.
So, Carmarthen stands as the only loss of a by-election by the Liberals since Lambeth North in 1934. The Labour candidate who captured Lambeth North from the Liberals was George Strauss, the son of a Conservative who had defected to the Labour Party. Having won the seat, Strauss himself was temporarily expelled from the Labour Party in 1939.
In terms of time, Eastleigh is a long way from Carmarthen, and even further from Lambeth, and there were no high profile defectors of Megan Lloyd George’s reputation waiting in the wings to try and take Eastleigh from the Liberal Democrats.