Some Iowa Republicans are frustrated with their state party chairman, A.J. Spiker, for changing the GOP’s nominating convention date — and at least one top local operative has called for his resignation.

By pushing back the party’s nominating convention by a month, Democrats will have an advantage in a rare open Senate race, Republicans argue. Spiker has called a Sept. 23 meeting for the state central committee to address the convention date, according to The Iowa Republican blog. CQ Roll Call confirmed the meeting.

In Iowa, candidates must receive 35 percent of the vote in a primary or delegates decide the nominee in an unpredictable convention process. (Read Roll Call’s primer on the situation here.) At least six Republicans have announced bids to succeed retiring Democratic Sen. Tom Harkin, almost ensuring a nomination convention will be held in the Hawkeye State for the first time since 2002.

Originally, the state party scheduled its nominating convention for June 14, less than 10 days after the primary. But Spiker pushed the convention date to July 12.

Prominent Iowa Republicans, including Sen. Charles E. Grassley and Gov. Terry E. Branstad, have expressed frustration with the switch. Republicans argue this gives Rep. Bruce Braley, D-Iowa, an advantage in the race. Braley is widely expected to be his party’s nominee.

After Spiker refused to reconsider his decision, a top local GOP operative, David Kochel, called for his resignation on the radio.

“The argument A.J. is putting forward is ridiculous,” Kochel said, according to the Des Moines Register. “I think it’s time he resigns. He should resign immediately from the chairmanship.”

The Iowa Senate race is rated a Lean Democrat contest by Rothenberg Political Report/Roll Call.