After dropping out of the Presidential race, Ted Cruz filed for reelection to the Senate in 2018.
You would think this is pretty early, but at least three senators, Democratic Bill Nelson of Florida, Independent Angus King of Maine (who caucuses with Democrats) and Democrat Joe Manchin of West Virginia), have also announced reelection bids. Given that Manchin is probably the last Blue Dog Democrat in the senate, I wouldn’t be shocked to see him cross the aisle to join the Republicans between now and 2018.