Lt. Gov. Bill Halter (D) forced Sen. Blanche Lincoln (D) into a June 8 runoff after both failed to win a majority of the vote in Tuesday's Senate primary.
Lincoln led Halter 44 to 42 percent with more than half of the state's precincts reporting. The Associated Press projected the race was heading to a runoff.
Halter's campaign considered Tuesday a victory, noting that he won three counties in Lincoln's old congressional district.
On the GOP side, Rep. John Boozman (R-Ark.) appeared headed toward the GOP nomination. He led a field of eight candidates with 51 percent of the vote with 1,458 of 2,516 precincts reporting.
Halter's candidacy was backed by a coalition of progressive groups and labor unions angered by Lincoln's perceived rejection of the Obama administration's agenda.
His union allies said Tuesday's result should send a "clear message."
"Working families all over the state of Arkansas have sent a clear message: they will not stand up for those who fail to stand for them. What Senator Lincoln learned tonight and what elected leaders all around this country should note is that the days of 'business as usual' are over," SEIU President Mary Kay Henry said in a statement.
AFL-CIO President Richard Trumka vowed to press on.
"From the minute the polls closed tonight, we began our renewed efforts to elect Bill Halter to the Senate in the runoff election," he said in a statement.
Updated at 11:37 p.m.