A Wall Street Journal report at the link below says:
Democrats for the first time are acknowledging that Republicans could retake the Senate this November if everything falls into place for the GOP, less than two years after Democrats held a daunting 60-seat majority.
A change of party control in the Senate, where only a third of the members are running and Republicans must capture 10 seats, seemed out of the question.
That's no longer the case. The emergence of competitive Republican candidates in Wisconsin, Washington and California—Democratic-leaning states where polls now show tight races—bring the number of seats that Republicans could seize from the Democrats to 11.
More details here.