一瀉千里 (いっしゃせんり) is a 四字熟語 meaning 'in one swift rush' — literally 'one pour, a thousand miles.' 党首 (とうしゅ, 'party leader') is political vocabulary. 述べ立てた (のべたてた, 'stated forth, declared at length') is the compound verb. The 四字熟語 conveys speed and unstoppability — the speech poured out like a river.