Retiring Sen. Ben Nelson (D-Neb.) will not endorse former Sen. Bob Kerrey (D-Neb.) in a Democratic primary. 

"I usually don't take sides in contested primaries, so that's pretty much where I am," he told The Hill on Tuesday. 

Kerrey has long been national Democrats' preferred choice, but after he initially said he wouldn't run, Nebraska Regent Chuck Hassebrook (D) entered the race.

Kerrey changed his mind and jumped in after Senate Majority Leader Harry ReidHarry Mason ReidDon’t worry (too much) about Kavanaugh changing the Supreme Court Dem infighting erupts over Supreme Court pick The dishonesty of the deep state MORE (D-Nev.) and other Democrats, worried about losing the seat, lobbied him hard. 

But Hassebrook has refused to step aside and clear the way for an uncontested primary.

Democrats face a steep uphill race to hold onto the seat in the heavily conservative state, and polls have shown both Kerrey and Hassebrook trailing their GOP opponents by wide margins. But national Democrats are more confident that Kerrey can make the race competitive.

Nelson said he'll help out whoever wins the nomination, and downplayed the impact of a contested primary.

"I'm close to both of them, and I'll actively campaign when we get past the primary," he said. "We're only talking a couple of months."