WASHINGTON — The still-emerging Tea Party movement is not merely waging war against the Republican establishment this year. Some of its more heated disputes are coming from within.

That struggle is playing out vividly in Nebraska, which will hold a Republican Senate primary on Tuesday between former State Treasurer Shane Osborn and Ben Sasse, the president of Midland University.

Mr. Osborn, who has the support of activists in the state, secured a major endorsement last November from FreedomWorks, the organization that helped vault the Tea Party to prominence. Mr. Osborn, the group said, stood “with the grass-roots uprising before it was cool.” But in March, FreedomWorks rescinded its support of Mr. Osborn and backed Mr. Sasse.

Ever since, Nebraska’s Tea Party members have been battling national Tea Party donor groups.

“We are not million-dollar Washington, D.C., special interest groups with strong ties to Capitol Hill. We are simply Nebraskans who are fed up,” a group of 52 activists wrote in an open letter protesting FreedomWorks’s about-face, adding, “We were not consulted, polled, or contacted by these Washington, D.C., groups.”