North Dakota's Democratic Sen. Heidi Heitkamp faces re-election in a state President Donald Trump won by more than 30 points in 2016. In Nevada, Republican Sen. Dean Heller will try to defend his seat in a state Democrat Hillary Clinton narrowly won in 2016.
Both elections are crucial in determining the Senate's party breakdown following November's midterm elections. Republicans aim to keep or expand their 51 to 49 seat majority in the chamber.
The GOP's ability to do so will have major implications for which proposals the chamber will pursue next year. It will also affect Trump's ability to get political and judicial nominees approved.
While the political environment appears to favor Democrats this year, the party faces an uphill battle to gain seats in the Senate. Twenty-six Democratic incumbents and independents who caucus with them face re-election this year. Only nine Republican seats are up for grabs this year.
While Heitkamp and Heller's seats are two of the main events this year, voters on Tuesday will also choose a Republican challenger to Sen. Tim Kaine, D-Va. Kaine is considered relatively safe in the blue-leaning state.