The NF fielded 45 candidates in the 1969 local elections; they averaged a vote share of 8%, although a few secured over 10%. The NF focused on these latter seats in the 1970 local elections, fielding 10 candidates: almost all received under 5% of the vote. The party had faced militant left-wing opposition, including a lorry that was driven into its Tulse Hill building in 1969, and to counter this the NF installed a spy in the London anti-fascist movement. Against Chesterton's wishes, NF activists carried out publicity stunts: in December 1968 they marched onto a London Weekend Television show uninvited, and in spring 1969 assaulted two Labour Party ministers at a public meeting, thus accruing a reputation for rowdiness. While Chesterton was holidaying in South Africa, a faction led by Gordon Brown—formerly of Tyndall's GBM—launched a leadership challenge against him. On realising that his support was weak, Chesterton resigned. He was succeeded by John O'Brien in February 1971. O'Brien and his supporters were frustrated that Tyndall and his associate Martin Webster maintained links with neo-Nazi groups like the Northern League; O'Brien unsuccessfully tried to expel Webster from the party. After this failed, O'Brien and his allies left the NF and joined John Davis's National Independence Party in June 1972.