Complex sentence: Finally, Claude Pepper declared his intention to challenge Truman for the presidential nomination; his candidacy was cut short when the Americans for Democratic Action and the Congress of Industrial Organizations withheld their support, appalled at his support for the Soviet Union.
Split into simpler sentences: Finally, Claude Pepper declared his intention to challenge Truman for the presidential nomination. His candidacy was cut short when the Americans for Democratic Action and the Congress of Industrial Organizations withheld their support, appalled at his support for the Soviet Union.