The Scottish Liberals now possessed that most valuable of political commodities: momentum. Just six weeks after the 1964 general election, the incumbent Unionist MP for Roxburgh, Selkirk and Peebles (a seat briefly held by a Liberal in the early 1950s), the elderly and ineffective Commander Charles Donaldson, died during surgery in London. The Times judged that the resulting by-election gave the Liberals a chance to further increase their representation in the Commons, noting that David Steel, ‘a young and vigorous campaigner’, had obtained the third-highest Liberal vote among the 54 Liberals who had achieved second place at the recent election.

Steel had been what Jo Grimond's biographer called the ‘beneficiary of considerable sponsorship’ from the Liberal leader. Indeed, the two were close personally (Steel had been Grimond's bag carrier during the campaign) and ideologically. Russell Johnston, a university contemporary, later observed that ‘David was always a Grimond Liberal’, whereas he and Judy Steel, David's wife, were ‘Bannerman Liberals’, a reference to SLP folk hero John Bannerman, who had failed to take Paisley at the 1964 election.

Grimondites were, crudely speaking, less passionate about Scottish Home Rule than followers of John Bannerman who, according to H. J. Hanham, ‘looked at times to be a Home Ruler who happened to be a Liberal rather than vice versa’. George Mackie judged that Bannerman was ‘never anti-Scottish Nationalism’ but ‘highly suspicious’ of some in the SNP. Jo Grimond's ‘heart’, by contrast, was in Mackie's view ‘unquestioningly in Westminster and in Orkney and Shetland’. ‘It was very difficult to get Jo to talk about a Scottish Parliament,’ Mackie added in his memoirs, ‘although he would talk happily about devolution.’

This division – or more accurately, difference of emphasis – informed ongoing talk of a Liberal-SNP pact, which loomed large for the remainder of the 1960s. The SNP's suggestion of a pact had been rebuffed on the grounds that the SLP, ‘although autonomous, was part of the Great Britain Liberal Movement, and would not contemplate any unilateral action for Scottish self-government’. David Steel, the Scottish Liberal Party's assistant general secretary, reckoned the pending Borders by-election could also be a ‘test case’ for Grimond's ‘realignment’ strategy of closer co-operation with the Labour Party.