This paper considers the rise of UKIP and the mainstream parties' reactions to its stance on immigration. This paper accordingly seeks to examine the specific themes contained within the rhetoric of the mainstream political party leader speeches conveyed between September 2013 and December 2014 in order to ascertain the underlying messages being employed regarding immigration - a key UK 2015 election campaigning topic. This examination will entail a comparative analysis of speeches conveyed by the leaders of the Liberal Democrat, Conservative and Labour Parties, and UKIP. Combining two forms of discourse - Benoit's (2007) functional theory and Wodak and Meyer's (2015) analysis of ideology and political discourse - this study addresses the following research question: How have the mainstream political parties responded to UKIP's challenge on immigration as part of their political communication? The results of the analysis provide fresh insights regarding the use of message themes, namely, acclaims, attacks, defences, policy and character, in the treatment of the question of immigration by mainstream political parties, including UKIP.