Fife North East, which Jo Swinson visited on Friday, is the most marginal seat in the whole of the UK.
At the 2017 election, three recounts were necessary before the final result was declared.
Stephen Gethins, of the SNP, held the constituency by just two votes ahead of the Liberal Democrats.
It was a bitter result for the Lib Dems, who had previously represented the seat from 1987 to 2015.
Now the party has another chance to win it back – but the SNP will be equally determined to hold on.
Both parties got 33% of the vote in 2017, with the Tories in third place on 24%.
Labour finished a distant fourth on 10%.
The result this time could be affected by tiny shifts in support between the SNP, Lib Dems and Conservatives.
For example, if a small number of pro-Remain but anti-independence voters switched from the Tories to the Lib Dems, this could be enough to hand victory to the Lib Dems and defeat the SNP.
By contrast, the SNP might be able to exploit the closeness of the 2017 result as a way of encouraging more of their supporters to go to the polls, in turn boosting their chances against the Lib Dems.