He eventually joined the Republican Party and won election to the United States House of Representatives from Virginia's 9th congressional district and controlled federal patronage in the Commonwealth from 1903 to 1907.

[3][4] During the Civil War, Slemp volunteered for the Confederate States Army and joined Company A of the 21st Virginia Infantry Battalion, becoming the laters commanding officer.

The Cumberland Gap was initially fortified by Confederate General Felix Zollicoffer (a former Tennessee Congressman), and contested by Unionist Tennessean Capt.

Although Zollicoffer was killed in action in the January 1862 during the first Kentucky invasion, Union Generals George H. Thomas and later William T. Sherman had difficulty securing the Cumberland Gap.

At Pound Gap in nearby Wise County, Virginia, Slemp was among the Confederates opposed by Union Col. (later General and President) James Garfield, who forced their retreat.

When Union Major General George Morgan was sent to secure the Cumberland Gap, Colonels William M. Churchwell and James E. Rains pleaded for reinforcements but their pleas were ignored; so they destroyed the supplies and evacuated.

When the Union troops tried to secure the gap they likewise failed to receive reinforcements, so Confederate Generals Stevenson and Edmund Kirby Smith were able to retake it in September 1862 as Morgan evacuated into Kentucky.

The incident related to his moving captured wagons on that date, about two weeks after Union raiders had burned the 64th Virginia's camp near Jonesville.

He became politically involved in the Readjuster Party, aligning with former Confederate General William Mahone who was consolidating railroads in Virginia and Tennessee.