Complex sentence: The only national party that defends itself as a centrist party is Citizens, whose program tends to go both left and right ways, though it is seen as a left party by conservatives and as a right party by socialists voters.
Split into simpler sentences: The only national party that defends itself as a centrist party is Citizens; its program tends to go both left and right ways. This party, however, is seen as a left party by conservatives and as a right party by socialists voters.