In the partisan battle zone that is Washington, there is one conquest that could turn the fight decisively in Republicans ’ favor: winning 60 seats in the U.S. Senate.
Right now the Republicans holds a 52 to 48 majority. 8 more seats and it will be filibuster proof, meaning they’ll be able to get anything they want. The mid-terms are very important if the Democrats want to be able to keep the Republicans in check.
Source: Here’s why the 2018 Senate election will be crucial for President Trump and his Democratic foes – LA Times