Short Answer 
Senator Jon Tester, reelected in 2012, will be up for reelection in 2018 due to the six-year term structure of the U.S. Senate. This system leads to staggered elections, with approximately one-third of senators facing reelection every two years.
Step 1: Understand Senate Terms
U.S. Senate terms last for six years, which means that after being elected or reelected, a senator serves for that duration. Senator Jon Tester was reelected in 2012, marking the beginning of a new six-year term for him. This term structure is crucial to understanding when he will run for reelection.
Step 2: Calculate Election Year
To determine when Senator Tester will be eligible for reelection, we need to add six years to the year he was reelected. This calculation is: 2012 + 6 = 2018. As a result, he will be up for reelection in 2018, which aligns with the general election cycle for U.S. senators.
Step 3: Grasp Staggered Elections
The U.S. Senate employs a staggered election cycle, where approximately one-third of the senators are up for reelection every two years. This system ensures continuity and stability within the Senate, allowing for a rotating reelection schedule. Therefore, in 2018, the elections for those serving six-year terms like Tester will occur, affirming that he will indeed face voters in that year.