Romney has a total of 1,076 delegates, leaving him just 68 shy of the 1,144 delegates needed to win the nomination. Texas will have 152 delegates at stake in its primary.
Romney has been in general election mode for weeks, spending his time fund-raising and focusing on President Obama. Obama, who didn't face a major challenger for his party's nomination, won the primaries in Arkansas and Kentucky - but without unanimous support.
In Kentucky's closed primary, about 42 percent of registered Democrats who voted selected "uncommitted." In Arkansas' open primary, voters could select a ballot for either party, and unofficial returns showed a Tennessee lawyer, John Wolfe, drawing almost 42 percent on the Democratic side.
On the Republican side, Romney won Arkansas with 68 percent of the vote and Kentucky with 67 percent, according to unofficial returns.
Texas Rep. Ron Paul, former Pennsylvania Sen. Rick Santorum and former House Speaker Newt Gingrich were on the ballot in both states. Santorum and Gingrich have both suspended their campaigns. Paul announced last week that he would no longer compete in the primaries, though he is still trying to amass national delegates at state conventions.