When Lyndon Johnson was president, Senate Foreign Relations Committee chairman William Fulbright held his fellow Democrat to account with hearings that challenged Johnson's escalation of the undeclared war in Vietnam. It was the right thing to do.
When Ronald Reagan was conducting a lawless dirty war in Central America, Republicans such as Lowell Weicker of Connecticut and Charles Mathias of Maryland raised objections to the policies and actions of their party's president.
When Bill Clinton steered the United States into the conflict in Yugoslavia, Senator Russ Feingold and Congressman Dennis Kucinich rejected partisanship to demand that the Democratic president respect the constitutional requirement that wars be declared.
Even when George Bush and Dick Cheney were enforcing the strictest party discipline, Iowa Congressman Jim Leach co-sponsored a resolution of inquiry into whether his fellow Republicans had conspired to lie about the supposed "threat" posed by Iraq.
In every case, the members of the Congress rejected the party line in order to defend the rule of law, which requires in our system of separated powers that the legislative branch check and balance the executive. It wasn't personal. It was a matter of principle. In this, they accepted an understanding of the separation of powers articulated by then-Senator Barack Obama, who said in 2007: "The notion ... that the president can continue down a failed path without any constraints from Congress whatsoever is not warranted by our Constitution."