The constituency of St Albans, which Jo Swinson visited on Monday, is one of the Liberal Democrats’ top targets in the country.
A swing of 5.4% would see the Lib Dems take it from the Conservatives – the sort of swing that would hand the Lib Dems around half a dozen other Tory seats in southern England.
But to win St Albans they will need to defeat Anne Main, who has represented the seat for the Tories since 2005.
And Ms Main has seen off a number of attempts to beat her, including close races with the Lib Dems in both 2010 and 2017.
At the 2017 election she got 43% of the vote, while her Lib Dem challenger finished on 32%.
Labour came third with 23%.
St Albans was a three-way marginal during the 1970s and 1990s, and was held by Labour from 1997 to 2005.
This year it will almost certainly be a two-horse race, however, between the Tories and Liberal Democrats.
The Lib Dem candidate is Daisy Cooper, who stood for the seat in 2017.
Also standing this time are Labour, Green and independent candidates.
The Brexit Party has pulled out, but Brexit could still play a key role in the contest.
The area covered by the local authority of St Albans – which includes most of the St Albans parliamentary constituency – voted 62.7% Remain in the 2016 EU referendum.
This was the 29th highest Remain vote in the country.
Anne Main campaigned for Leave in the referendum – something the Liberal Democrats will hope counts against her on polling day, as it seemed to do at the 2017 election when there was an 8.7% swing in St Albans from the Conservatives to the Lib Dems.
A similar swing at this election would see Ms Main beaten.