I chose Tester for the reason that Montana is a heavily Republican state
This is a popular myth, but is, nonetheless, a myth. The governor is a Democrat. Both senators are Democrats. The attorney general is a Democrat. The state auditor is a Democrat. The Montana Senate is Democratic. The Montana House of Representatives is one seat away from being Democratic.
The only notable statewide elected positions that are held by Republicans, in fact, are the secretary of state and the representative of the at-large district. Everything else is Democratic.