Murder in the White House (Capital Crimes Series, Book #1) by Truman, Margaret - 1980

In a town where the weapon of choice is usually a well-aimed rumor, the strangling of Secretary of State Lansard Blaine in the Lincoln Bedroom is a gruesome first. White House counsel Ron Fairbanks is ordered to investigate. There are persistent rumors that the Secretary was an accomplished womanizer with ties to a glamorous call girl. There is also troubling evidence of unofficial connections with international wheeler-dealers.

In death as in life, Blaine is a power to be reckoned with. For Fairbanks, who loves the president's daughter, on point is soon clear: only a few highly placed insiders had access to the Lincoln Bedroom that fateful evening. And one of them was the president.