Everything. Block any bills they don’t like, make the budget they want, control every committee, subpoena evidence against Trump, call hearings, depose witnesses. They can’t pass their own legislation because they don’t have the Senate or a veto-proof House majority, but other than that, the sky’s the limit.
The US pretending to bring democracy to other countries is so funny when we vote on a non-holiday workday where people can wait up to six hours for a machine that may record their vote incorrectly to defeat a minority party that controls all three branches of government.