Next month’s midterm elections will be the most costly congressional races in US history, with business interests contributing three-quarters of the estimated $2.6bn that will be spent, says a non-partisan research group.

In a report to be released on Wednesday, the Center for Responsive Politics predicted that Republican candidates, parties and advocacy groups would spend $1.4bn (€1.1bn, £746m) while Democratic interests would spend $1.2bn.