The ruling party forced the tax reform bill through.

Literal

Ruling-party [topic-は] forcibly tax-system-legislation [object-を] pass-cause.

強引に ('forcibly / high-handedly') is a adverbial with strong negative connotations. 通過させる is the causative of 通過する ('pass through') — 'cause to pass,' i.e. forced it through. 税制法案 ('tax system legislation').