Cash, a social conservative activist who narrowly lost a House primary runoff election to now-Rep. Jeff Duncan (R-S.C.) in 2010, said Graham has lost touch with South Carolina values.

"I am running to replace Senator Lindsey GrahamLindsey Olin GrahamHouse Judiciary chair threatens subpoena if DOJ doesn’t supply McCabe memos by Tuesday Rosenstein report gives GOP new ammo against DOJ Graham: There's a 'bureaucratic coup' taking place against Trump MORE because I believe that after 20 years in Washington he is out of touch with South Carolina values and voters. Although Senator Graham might be hard-working and sincere, I cannot distinguish his core principles and can no longer trust his judgment," he said on his website.

"It is time for a new voice in Washington. We need a voice that represents Christian, conservative, and Republican principles instead of the latest bi-partisan 'deal' that simply makes matters worse."

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Cash said "the origins and foundation of American greatness lies in Christianity, Capitalism, and the Constitution" and promised he was "unapologetically committed to faith and family as the essential elements of enduring freedom."

Polling shows Graham is in a fairly strong position for reelection. But some on the right are unhappy about his bipartisan work on a number of issues, such as immigration reform. Libertarians in the party don't like Graham's foreign policy views and are still furious about his attacks on Sen. Rand PaulRandal (Rand) Howard PaulA Senator Gary Johnson could be good not just for Libertarians, but for the Senate too Conservatives left frustrated as Congress passes big spending bills Senate approves 4B spending bill MORE (R) following the Kentucky senator's talking filibuster of President Obama's CIA director nominee, John Brennan.

Other Republicans are mulling a challenge to Graham as well, including state Sen. Lee Bright (R) and Paul backer Nancy Mace.