Another top Democratic contender for Michigan’s open Senate seat has begun informing colleagues she will take a pass on the race, according to three people familiar with the conversations, narrowing the field for a seat that Democrats are increasingly worried about whether they can keep next November.Rep. Kristen McDonald Rivet, a freshman Democrat who won a key battleground seat last year, has decided to stay in the House – a decision that is making some Michigan Democrats anxious about their chances of securing a top-tier candidate to go up against a Republican who nearly won the seat last year.***The fate...