Bob Woodward's forthcoming book on President Trump contains an allegation that is causing heartburn for congressional Republicans. I can confirm from conversations with several senior members of Congress that the GOP thinks this could be what actually triggers an inquiry into "high crimes and misdemeanors."
According to Executive Order 12333, "No person employed by or acting on behalf of the United States Government shall engage in, or conspire to engage in, political assassination." Now, you can argue that if a President orders an assassination, he has reversed the Executive Order. But there is an actual process by which an Executive Order is reversed. The President cannot do it on a whim.
In Woodward's book, he says President Trump ordered the Secretary of Defense to prepare a plan to assassinate Bashar al-Assad, the Syrian dictator. The Secretary of Defense, according to Woodward and also according to people close to Secretary Mattis, refused to go down that road. To go down that road would have not just violated the executive order, but would have also most likely bring a direct military conflict with Russia.
Democrats are, behind the scenes, increasingly skeptical of the Russian collusion story largely because they think the public is not really sold and also because they increasingly think Bob Mueller is not going to directly connect the President. That would hurt them building a public case.
But they seem more convinced they can build a case of an erratic President randomly ordering the assassinations of world leaders he does not like even if that might mean starting World War III. And they'd love to put Mattis under oath and see the White House either assert executive privilege or haver Mattis verify the allegations. They think just having Mattis there before the world stage would give them what they need to move public opinion towards impeachment.
And privately, if the Democrats actually hold their own or make gains in the Senate, they already have around 15 Republicans who would gladly help them impeach Donald Trump.
I do think, however, that what Democrats miss is that Assad is a war criminal and President Trump ordered the assassination attempt after Assad used chemical weapons on his own people. I think the President may be able to take the high ground on this.