Republicans are trying to sink the leading Democrat in Nebraska’s most competitive congressional race by tying him to… Donald Trump. The state’s 2nd congressional district is a top pickup opportunity for Democrats this cycle, given the retirement of five-term G.O.P. Rep. Don Bacon, and the party is embroiled in a competitive three-way primary there. But American Action Network, the dark-money group closely aligned with the Congressional Leadership Fund and Speaker Mike Johnson, has been targeting the current favorite, Democratic State Sen. John Cavanaugh, as a MAGA Trump ally—a potentially crippling association in a Democratic primary.