The key US Senate races that could determine who controls the chamber

Democrats on defensive to keep narrow majority with Montana and Ohio races as Republicans eye swing states

The Democrats are fending off a Republican offensive aimed at overturning their tiny majority in the Senate.

Retaking control of the US Congress's upper chamber may represent the GOP's best opportunity of success in November's election, according to analysts, surpassing their chances of retaking the White House or even retaining control of the House of Representatives.