Before I get to the presidential analysis, I just wanted to give this little historical tidbit: Arkansas didn’t vote to secede at first in early 1861; it did so after Lincoln ordered their troops to Fort Sumter to stop the rebellion.
Being a Southern state, Arkansas naturally remained solidly in Democratic hands in the first half of the 20th century. The northwest, a Union-supporting region, had been the center of Republican strength in the state. The poor-quality soil of western Arkansas and resultant small numbers of slaves made this part of the state pro-Union/Republican. Soil in the east of the state, in the Arkansas Delta is rich and fertile thanks to sediment deposits from the Mississippi River. This soil was suited for growing cotton and later rice.
Later Republicans expanded their strength to the southwest (Texarkana), in the 50s and the northeast (Jonesboro) and Little Rock after the Civil Rights Act passed. This point in history also saw the elections of John Paul Hammerschmidt, the first post-Reconstruction Republican congressman, and Winthrop Rockefeller to the governorship. Rockefeller managed to win with a coalition of “progressive Democrats and newly enfranchised black voters”, though he lost in 1970 to progressive Democratic challenger Dale Bumpers. Rockefeller expected to face the infamous Orval Faubus.
Rockefeller’s coalition probably explains why Arkansas held out for Democrats as long as it did. The Arkansas Republican Party was weakened by Rockefeller’s death in 1973, and the state went heavily for Jimmy Carter. It did go for Reagan in 1980, but barely. And of course when Bill Clinton ran for President, Arkansas went strongly for him, being the only state to give a majority of its votes to a presidential candidate in 1992.
The Republican trend picked up after the Clinton years, solidifying the northwest as a Republican stronghold, and also turning north central Arkansas, in the Ozarks, red as well. Southern and eastern Arkansas remained Democratic then, with the bottom falling out in the late 2000s. The Obama years and after saw Republicans solidify their grip on most of the state. Northern Arkansas, in and around Mountain Home, and western Arkansas around Fort Smith are the most Republican parts of the state, giving Republicans 70%+ of the vote. The Democratic areas are in Little Rock, the Mississippi Delta, and Jefferson County (Pine Bluff).