Then-congressman Lindsey Graham, R-S.C., shown in this video image from the Senate floor on Jan. 16, 1999, tells the Senate what a difficult decision they have to make during the third day of opening statements in the impeachment trial of then-president Bill Clinton.
Graham shakes hands with Sen. John McCain, R-Ariz., after McCain's speech to the South Carolina Policy Council in Columbia, S.C., on April 19, 2000.
Then-president George W. Bush stands with Graham on the steps of Air Force One at the Greenville/Spartanburg Airport on March 27, 2002, in Greenville, S.C.
Graham addresses the Republican National Convention at Madison Square Garden in New York City on Aug. 30, 2004.
Then-senator Joe Biden makes a point with Graham as they appear on "Meet the Press" on Jan. 7, 2007, at the NBC studios in Washington.
Graham addresses delegates at the Republican National Convention in St. Paul, Minn., on Sept. 4, 2008.
Graham joins McCain at a campaign rally at Pennridge Airport on Nov. 1, 2008, in Perkasie, Pa.
Graham listens during the Judiciary Committee's confirmation hearing for Supreme Court nominee Elena Kagan on June 29, 2010.
Graham takes part in a Senate Judiciary Committee hearing on gun violence on Jan. 30, 2013.
Graham talks to reporters after leaving a Senate Republican caucus meeting at the Capitol on Oct. 11, 2013, during the government shutdown.
Graham speaks to supporters after winning his Senate primary on June 10, 2014, in Columbia, S.C.
Sen. Benjamin Cardin, D-Md., left, listens as Graham speaks during their appearance at the American Israel Public Affairs Committee Policy Conference in Washington on March 1, 2015.
Graham fields questions from reporters at the Iowa Ag Summit on March 7, 2015, in Des Moines.
Graham speaks at the Republican Leadership Summit on April 18, 2015, in Nashua, N.H.
In this handout image provided by the Government Press Office, Israel Prime Minister Benjamin Netanyahu shakes hands with Graham during a meeting at his office on May 27, 2015, in Jerusalem.
Graham announces his presidential candidacy in Central, S.C., on June 1, 2015.
Graham plays a game of pool following a campaign stop at Veteran of Foreign Wars Post 1623 on June 5, 2015, in Waterloo, Iowa.
Graham, Sen. Tim Scott, R-S.C., and Rep. Mark Sanford, R-S.C., attend a memorial service for the victims of the shooting at Emanuel AME Church on June 19, 2015, in Charleston, S.C.
Graham gets a fist bump from Glenn Bacon during a campaign stop at Milly's Tavern on July 10, 2015, in Manchester, N.H.
Graham speaks at the Family Leadership Summit in Ames, Iowa, on July 18, 2015.
Graham speaks during a visit to the Iowa State Fair on Aug. 17, 2015, in Des Moines.
Graham speaks at a "Stop Iran Rally" regarding the Iran nuclear deal on Sept. 1, 2015, in New York City.
Graham throws his hands up after losing while playing the roulette wheel at a charitable gaming poker room during a campaign stop at the Manchester Bingo Center and Poker Room on Oct. 9, 2015, in Manchester, N.H.
Graham is seen gesturing on a television inside the spin room during the CNBC debate at University of Colorado's Coors Events Center on Oct. 28, 2015, in Boulder.
Graham speaks about the threat America faces from radical islamic movements at the Metropolitan Republican Club on Nov. 19, 2015 in New York City.
Graham gestures on stage during the undercard debate, hosted by CNN, at The Venetian hotel in Las Vegas on Dec. 15, 2015.