2021 Chesham and Amersham by-election

Chesham and Amersham, named after the market towns of Chesham and Amersham in the constituency, was held by the Conservative Party since its creation at the February 1974 election until the by-election. The local authority is Buckinghamshire Council and the seat is coterminous with the abolished Chiltern District. The area is in the London commuter belt, and is home to many affluent professionals.[8]

In June 2016, 55% of voters in the coterminous Chiltern District voted to remain in the European Union (EU) in the EU membership referendum. The estimated turnout of 83.6% was the highest figure for any constituency in the UK, with only Gibraltar reporting a higher percentage.[9] At the 2019 European Parliament elections, the pro-EU Liberal Democrats were the most popular party in the Chiltern District with 31.9%, with the pro-Leave Brexit Party in second place with 30%.[10] Despite the seat voting Remain, Gillan, a supporter of Brexit, was re-elected with over half the vote in both the general elections following the referendum. Following the 2019 election, it was the Liberal Democrats' 51st target going into the next election, requiring a swing of 14.6 to win.[11]

• Liberal Democrats – Sarah Green, a training and communications professional.[12][13] Green was the party's candidate in Ynys Môn in the 2005 general election, coming fifth with 6.8% of the vote (while serving as Chair of MIDR Cymru (now Welsh Young Liberals)), and Arfon in North Wales in 2010, coming fourth, with 14.1% of the vote.
• Conservative Party – Peter Fleet, Chairman of the Retail Automotive Alliance and former president of Ford Asia Pacific.[14][15] Fleet was the Conservative candidate for Southampton Itchen in the 1997 general election, and came second with 28.4% of the vote.[16]
• Green Party – Carolyne Culver,[17] Green Party group leader on West Berkshire Council.
• Breakthrough Party – Carla Gregory,[18] a charity worker from the local area.[19]
• Labour Party – Natasa Pantelic, elected member of Slough Borough Council,[20] and its lead member for health and wellbeing.[21]
• Freedom Alliance – Adrian Oliver, former member of the Green Party, elected as a councillor for the Greens in Camden in the 2006 election.[22]
• Reform UK – Alex Wilson, who previously worked for HS2 Ltd.[23]
• Rejoin EU – Brendan Donnelly, a former Conservative Member of European Parliament.[23]

On 9 May, former Conservative MP Phillip Lee, who joined the Liberal Democrats before the 2019 election, urged the Labour and Green parties not to stand candidates in the by-election to avoid vote splitting. Lee argued that the Conservative Party would be more likely to lose if the anti-Conservative vote went to the Liberal Democrat candidate alone.[24] Writing in The Guardian on 14 May, columnist Polly Toynbee also called for an electoral alliance of the Liberal Democrats, Labour and the Green Party to beat the Conservatives.[25] However, all three parties contested the by-election.

On 12 May, the writ for the by-election was moved in the House of Commons by Government Chief Whip Mark Spencer.[26][27]

On 30 May, The Independent reported that internal party polling by the Liberal Democrats put the Conservatives on 45.5% and the Liberal Democrats on 35.1%, a swing of 9.35% to them. The same survey also found 60% of Labour and Green voters were willing to vote tactically.[28] Further internal party polling by the Liberal Democrats, reported by City A.M., found a closer margin, with the Conservatives on 45% and the Liberal Democrats on 41%. Despite these figures, YouGov's director of political research said "the by-election will be tough to win for the Lib Dems" due to the party putting itself up against a "popular government", unlike when it had won by-elections in the past.[29] Lib Dem leader Ed Davey said that he visited the constituency 16 times during the campaign.[30]

The results shocked the political landscape, as politicians and pundits had predicted a Conservative win in the run-up to the by-election.[32] John Curtice, Professor of Politics at the University of Strathclyde, expressed the view that the Liberal Democrats overperformed and compared the by-election to the 1991 Ribble Valley by-election, in which the Liberal Democrats gained a Conservative seat because of local issues. He relayed some locals were "hearing the Conservatives talking endlessly, talking about 'levelling up' – wondering really, well what is, quite, in it for us?"[33] Some commentators suggested the result may indicate wider difficulties for the Conservative Party across the south of England, drawing comparisons to the Labour Party's problems holding on to many of its older bases of support.[34][35][36]

This was the lowest ever share of the vote in a Westminster election by a Labour Party candidate.[33]