Maricopa County Attorney Bill Montgomery on Wednesday called on Tom Horne to step down as attorney general. That makes him the first elected official from Horne’s own Republican Party to actually go that far.

Montgomery conceded he’s backing Mark Brnovich in the GOP primary against Horne. But Montgomery insisted his call, which started a Twitter war with a Horne aide, is in the best interests of the state, saying he has “a lack of confidence in his ability to lead an organization of that size.”

Horne noted that Montgomery’s 2013 investigation into Horne’s 2010 political activities was sidelined by a judge who said the county attorney had no jurisdiction .

“He is losing relevancy as demonstrated by engaging in irrational adolescent Tweets against my staff rather than doing his job,” Horne said in a prepared statement. “Perhaps Bill Montgomery is the one who should resign.”

Several Republicans have previously called on Horne to abandon his bid for re-election amid fears that a divisive GOP primary — one Horne might win — would lead to a victory in November by Democrat Felecia Rotellini. Horne narrowly defeated her in 2010.

That race continues to hang over Horne’s head. First Montgomery and, after he was disqualified, Yavapai County Attorney Sheila Polk concluded that Horne violated campaign finance laws by coordinating his election bid with an independent committee.

Horne has appealed that conclusion to Maricopa County Superior Court Crane McClennen, pointing out that an administrative law judge actually found insufficient evidence of collusion.

But Montgomery said that ongoing legal battle is just part of the issue.

Horne is facing complaints by another former staffer that he is currently using his state office and employees in the re-election campaign.