Dutch Prime Minister Mark Rutte will have to hunt for new allies in the Senate after provincial elections further weakened his ruling coalition's tenuous grip on Parliament's upper house.

According to results of provincial elections compiled Thursday by Dutch media, Rutte's Liberals will remain the biggest party in the Senate but an ad hoc five-party bloc he relies on to pass legislation and budgets will no longer command a majority.

Aldermen elected in provincial elections Wednesday will vote in May for a new 75-seat Senate. If, as expected, they vote along party lines, the Liberal Party is forecast to take 13 seats and junior coalition party Labor will win just eight seats.

Rutte says, "We will have to work hard to find support" in the Senate.