U.S. Rep. Tom Cotton, R-Ark., the Republican candidate for U.S. Senate in Arkansas, waits to greet people entering a polling place on Nov. 4, 2014 in Little Rock, Arkansas. Justin Sullivan/Getty Images

Republican challenger Rep. Tom Cotton ousted Democratic Sen. Mark Pryor in Tuesday’s crucial midterm election. Arkansas is the second state to see a Democratic senator lose a seat to a Republican in the 2014 elections. Republicans need four more seats to take control of the U.S. Senate.

Cotton had consistently polled ahead of Pryor since July, but the race was considered close until near the end. Cotton led Pryor by 8 points going into Election Day, adding to predictions the Senate would shift from Democratic to Republican control for the first time in eight years. Election Lab, LEO and FiveThirtyEight all gave Republican candidates in Arkansas a 70 percent or higher chance of winning leading up to the race.

Cotton's victory means both Arkansas U.S. Senate seats are controlled by Republicans. The other U.S. senator is Republican John Boozman.

Cotton served as the representative for Arkansas’ 4th Congressional District for one term following Democratic Rep. Mike Ross’ retirement. He is staunchly conservative and caught some criticism for voting against the federal aid package for victims of Hurricane Sandy, which he said was laden with excessive spending “not related to the disaster at all.” Pryor attacked him relentlessly for it in his campaign ads.

Arkansas is considered a red-leaning state, but anchored less on ideology and more on practicality, according to outgoing Democratic Gov. Mike Beebe. “Some of those pragmatic business Republicans see more advantage to Senator Pryor’s re-election than they do to an extremely, very, very conservative opponent,” he said.

Cotton focused on Pryor’s support for President Barack Obama and his vote in favor of the Affordable Care Act, or Obamacare, which is unpopular with Arkansans. Former President and Arkansas Gov. Bill Clinton crisscrossed his home state in support of Pryor and gubernatorial candidate Ross, while former Gov. Mike Huckabee campaigned for Cotton and GOP gubernatorial candidate Asa Hutchinson, also a former congressman.

Pryor was a two-term senator first elected in 2002, when he unseated Republican incumbent Tim Hutchinson, Asa's brother. Hutchinson served one term, succeeding Pryor's father, David Pryor. In 2008, Mark Pryor won an 80 percent to 20 percent landslide against his only opponent, Green Party candidate Rebekah Kennedy. He served as a state representative for Arkansas’ 57th District from 1991 to 1995 and Arkansas attorney general from 1999 until his inauguration as senator.

The candidates each spent nearly $5 million in the race, but Republican interest groups spent $18.1 million to their Democratic counterparts’ $12.5 million, according to Smart Media Group.