Voters like to "balance" their representation. If left on president, then right on Congress. And vice versa.
There's every reason to believe that a nation choosing a president well to the left of mainstream might at the same time choose to ADD Republicans to Congress. This holds ESPECIALLY for those independents who might nervously vote left for a Bernie not out of conviction but because they saw the GOP presidential candidate as even less acceptable. A full third of the electorate now identifies as "independent."