Some researches have made efforts to analyze political debate based on the theories of Sax。 A research on 2012 US general election was done in a qualitative method to investigate the conflict talks between Barack Obama and Willard Romney。 (Jiao, 2014)。 However, mere analysis on opposing talks through transcription is not enough to conclude definitive features of debate。

2。1 Critical discourse analysis 

Critical Discourse Analysis(CAD) was first purposed by Roger Fowler, as a branch of instrumental linguistics in Language and Control in 1979, which aims to expose the manipulation and abuse of power in the political and social context and the continual use of discourse in order to establish and maintain the situation。 Exponents of CAD believe that ideologies universally exist and affect languages。 As the carrier of ideology, languages are the crucial means to command the power of discourse and society。 Based on the systematic functional grammars founded by M。K。 Halliday, CAD also absorbs Marxism from the West and emphasizes language functions in social structures and ideologies。

Each year there are extensive studies on presidential debates in the approach of critical discourse analysis。 Such 2004 presidential debate between George Bush and John Kerry（Li, 2005）, 2008 debate between Barack Obama and John McCain( Wang, 2010), 2012 debate between Barack Obama and Willard Romney（Zhao, 2013）。 This approach is effective in analyzing both the implicit and explicit information of verbal language。 However, nonverbal aspect of the debates is solely neglected, which weakens the soundness and completeness of the political discourse analysis。

2。2 Conceptual metaphor analysis

Lakoff & Johnson thinks conceptual metaphor analysis different from traditional theories of metaphors: The use of metaphor is not only a form of linguistic phenomenon, but a cognitive phenomenon demonstrating ways of thinking。 “The conceptual system on which we rely to think and act is in its nature metaphorical。 Professor Shu Dingfang holds the view that metaphors are the base for conceptual system of human organizations。 and what behind metaphors is a deep and complicated human cognitive behavior。 (Shu, 2000)

Mark Johnson (2003) points out that metaphor is a form of structure of human understanding, originating from the sensory movement experience from our body, which becomes the foundation of mental abstraction and reasoning。 From this perspective, truth is formed when connections between our body and the understanding of the situation consists with each other。 If consists, this statement is true。

Politicians employ metaphors in their construction of political discourse so as to achieve their political agendas。 In order to achieve their purpose, they use metaphors which are more familiar and acceptable to the general public to define their meaning。 In this way, the public will be easier to adapt to and even absorb the metaphors emotionally。 Thus conceptual metaphor analysis is a good way to reveal politicians’ inner world to the public。

Cognitive framework analysis is employed by some researchers to identify candidates debate strategies(Wang&Yuan)。 They believe metaphors, images and narratives are all crucial components in the candidate speech。 And cognitive framework is an important means to decode speaker’ intentions。(Ye, 2014) 文献综述

2。3 Multimodal Discourse Analysis

Although nowadays the number of researches on discourse analysis have grown along with the depth of studies。 However, if we only analysis discourse in the aspect of verbal languages, we cannot fully justify the complexity of discourse, in that a large proportion of discourse is embodied by non-verbal factors such as timbre, intonation, pace and body languages。 Apart from these, other environmental factors such as PPT, internet and lab also have parts to play。 (Zhang Delu, 2009) As the pioneer in multimodal analysis, R。 Barthes established a new branch for discourse analysis with his Rhetoric of the Image。 (1977) In his work, the interactive function between images and languages of meaning expression is investigated。 Following his lead, Kress, G。 & van Leeuwen, T。 (2001) studied the relationship between multimodal analysis and media, exploring the phenomena of expression including visual image, color grammar and newspaper column design and other different media functions。 They put forward the definition of modality: social cultural resources used for expression and communication through media which is shaped by society over time。 O’Halloran not only studied theoretical construction of multi-modal but also analyzed multimodal phenomenon in mathematical discourse。 (2004); Royce, T。 studied different signs in multimodal discourse in terms of complementariness and ESL classroom teaching。 (2002) Recent publication on multimodal discourse analysis reflects social semiotics’ influence on various modalities in different contexts。 (B& T 2006, Bateman 2008 Jewit 2009, Bednarek& Martin（2010）