20/05/2015 10:33 BST | Updated 20/05/2016 06:59 BST

Why Did 27 Lib Dem Seats Swing to the Conservatives?

Nick Clegg had predicted significant losses for the Liberal Democrats in the 2015 general election, but called the results "immeasurably more crushing and unkind" than expected as he resigned from the leadership on Friday morning. His party lost 48 seats, 15% of the popular vote and many of its grandees.

Clegg focused his campaign on the Lib Dem's centrism by promising to temper the impulses of what he had previously called "the siren voices on the Conservative right". His manifesto outlined a series of red lines for government, pledged to veto the Conservative's proposed £12 billion of cuts to the welfare budget and oppose legislation for increased surveillance powers.

But the message of acting as a moderator for Tory policies failed to speak to the electorate. Instead, the party ceded 27 seats to their former coalition partner, which gave the Conservatives an overall majority of 331 seats. Conservative gains included the Lib Dem strongholds of Twickenham, Lewes and Kingston and Surbiton, represented respectively by the former business secretary, Vince Cable, home office minister, Norman Baker, and energy secretary, Ed Davey.

Why did this number of former Lib Dem constituencies go blue? In 2010 65% of the seats won by the Lib Dems had the Tories in second place. Lib Dem supporters were split between what Clegg termed "virulently anti-Conservative voters" disillusioned with Labour, and centrist voters who broadly supported the Conservative's economic instincts but liked the Lib Dem's social justice agenda, and often had good relations with their local Lib Dem MP (see Ashcroft poll).

In this election, many voters who had defected from the Labour party to support the Lib Dems in 2010 returned to the Left. In Lewes, for example, they split their vote between the Green and Labour parties. But given that the Conservatives were the second largest party in Lewes and similar seats, a reduced Lib Dem vote gave Conservative candidates a majority.

After his defeat the former MP for Lewes, Norman Baker, bemoaned how erstwhile Lib Dem supporters wanting to prevent a Conservative majority had inadvertently put David Cameron back in Downing Street. "It is clear that up and down the country the seats the Tories won came from the Liberal Democrats," he told the Telegraph. "In seeking to punish us, [voters] have only succeeded in punishing themselves".

And just as a chunk of the Lib Dem's left-wing core moved to Labour and the Green Party, many of its Conservative-inclined voters from 2010 turned to the Right on election day. These Conservative "defectors" from the Lib Dems tended to support the Coalition, but credited the Conservatives with its achievements, focus groups show.

Yeovil was one such seat that swung to the right, with a 9.6% increase in the Conservative vote and losses of 22.6% for the Lib Dems. Across similar seats, hesitating Lib Dem supporters voted Conservative on election day, even if their first instinct had been to support another Lib Dem-Conservative coalition. This is because, says Nick Thornsby, a Lib Dem activist, they feared that a Lib Dem vote could translate into a Labour-led coalition in the event of Labour falling short of an overall majority but gaining more seats than the Conservatives.

"The voters that mattered most wanted David Cameron and George Osborne back in Downing Street," he wrote. "And the risk of voting Lib Dem and getting a Miliband government outweighed any desire to see a continuation of the Coalition".

The former Lib Dem home office minister, Jeremy Browne, gave a similar explanation when he predicted Lib Dem losses back in February. For him, the Coalition was a popular combination, but the Lib Dems disassociated themselves from its successes by publicly criticising Conservative policies. "The mistake the Lib Dems have made, given we were instrumental in creating the winning proposition, is to distance ourselves from it," he told the Independent.

"All we offer is a desire to water down [the Conservative's] strong views," he continued. "We offer an insipid moderation. Whichever party is the biggest one, we will stop them implementing a large number of their ideas. It is entirely negative".

In his resignation speech, Clegg did make the positive case for the Lib Dem's work in government, which Browne said had been so absent from its campaign. He championed his party's record on parenting rights, gay marriage, apprenticeships and raising the income tax threshold.

Clegg's replacement will want to emphasise this message of the Coalition's successes in order to win over Conservative voters. At the same time, the party will need to attract voters from the Left by rebranding itself and distancing its politics from those of the Conservative Party.

Tim Farron, a favourite for the leadership, suggested that a name change to "the Liberals" could help to remarket the party, but is also adamant that the Lib Dems must not turn its back on its achievements in government.

"We played a major part in turning the economy round," he wrote in an op-ed. "Every bit as important, we did what we could to make the last Government greener and more liberal than it would ever have been left to the Conservatives alone".