Nebraska GOP Senate candidate Shane Osborn raised over $550,000 in the first quarter of the year, his best fundraising showing yet.

That’s a significant improvement from his fourth-quarter haul of $369,000, and it marks 1,400 individual donations. He has about $660,000 cash on hand, however, an increase from the $582,000 he had in the bank at the end of last year.

Osborn, a former state Treasurer and decorated Navy pilot, launched his first ad last week, a positive spot that declares he’s the “true conservative” in the race and is the only one that can deliver on his campaign pledges.

His main rival in the four-way GOP primary for retiring Sen. Mike Johanns’ (R-Neb.) seat, Midland University President Ben SasseBenjamin (Ben) Eric SasseMeet the Democratic sleeper candidate gunning for Senate in Nebraska Longer sentences won’t stop the opioid epidemic Overnight Finance: Trump floats entering Pacific trade pact he once called 'a disaster' | Senators worry over Mulvaney's power at consumer bureau | Battle for CFPB control heads to appeals court | House fails to pass balanced budget amendment MORE, has not yet released his fourth quarter fundraising numbers.

But Sasse got a significant boost last week when national conservative group FreedomWorks withdrew its endorsement from Osborn and announced it would be backing Sasse instead.

The seat is expected to stay safely in Republican hands this fall.

—This piece was updated at 11:15 to correct Osborn's cash on hand figures.