With just a few primary elections to go, the competitive Senate map keeps shifting
- Democrats need four seats to flip the Senate; math still uncertain.
- North Carolina, Ohio, Maine, Michigan and Alaska are key toss-up races.
- Trump's unpopularity and economic strain give Democrats fresh openings.
With just a handful of primary contests left before November, the battle for control of the US Senate remains fluid, and Democrats need a net gain of four Republican-held seats to take the majority. Republicans expect to retain control but concede the margin could narrow, with Democrats potentially…