After Lincoln, I would say Nixon. The SDS and their offshoots, the anti-war protests and police riots, the Black Panthers in the aftermath of the King assassination, after Kent State he had the Weathermen…
Abroad:
Prime minister Aldo Moro in Italy had to deal with the Red Brigades who eventually kidnapped and murdered him after 55 days in captivity in 1978.
Helmut Schmidt, who just died last week, was the Social Democratic prime minister of West Germany from 1974 to 1982 had to deal with various violent leftist militant groups such as the Revolutionary Cells, the 2 June Movement, the Kommun, the Situationists, the Baader-Meinhof Gang which grew into the formidable Red Army Faction, and my favorite—the Socialist Patients’ Kollektiv (SPK)—a violent group of psychiatric patients-for-socialism who lived communally in a house just off the hospital grounds in Heidelburg. Their motto was “Our Illness is Our Weapon!” (raise fist defiantly).
. The militant student groups kidnapped various CEOs and their children, assassinated CEOs, bombed corporate headquarters and US military installations and generally raised hell. Schmidt had his hands full for sure. Not only this, but he had inherited a government rife with functionaries in key places who were former Nazi officials and current East German Stazi spies.