法案通過を推進しているロビイストは、運動資金に賄賂の金をまぎれこませました。

、。

The lobbyist pushing for passage of the bill slipped bribe money into the campaign funds.

Literal

Bill passage [object-を] promoting lobbyist [topic-は], activity-funds [into-に] bribe [possessive-の] money [object-を] slip-in-[made-ました]

Formidable sentence. 法案通過 is a compound: 法案 (bill) + 通過 (passage). 推進する ('to promote, push forward'). まぎれこませる is a causative of まぎれこむ ('to slip in among, become mixed in'), so the overall verb means 'to cause (the bribe money) to slip in'. The formal tone fits the political/journalistic subject.