Mehlenbacher was among a group of staffers who tendered their resignations amid the latest round of layoffs, which hit the operations team hard. The staff reductions and subsequent shifts have focused renewed attention on deep and long-standing dysfunction among the campaign’s top leaders.

Harris is trying to lift her candidacy from the low-single digits with a tenacious schedule that has her campaigning with family — and cooking — on Thanksgiving in Iowa. She has worked to navigate the internal dissension and improve her standing by correcting for mistakes that dogged her for much of the year, such as moving from defensive crouches to going on offense against her opponents on health care and her criminal justice record.

Yet while Harris has strung together a series of strong outings in recent weeks — from a well-received speech at the big Democratic dinner in Iowa to a solid debate performance — she has yet to see upward movement in polls. Meanwhile, she released a 50-second ad online that quickly went viral, generating 1.6 million views on Twitter, but has said she needs to raise money to put it on TV in Iowa where some leading candidates have been on the air for weeks.

Mehlenbacher’s departure for Bloomberg comes as his team cast a wide net in the days before and after the billionaire former New York mayor formally entered the race last weekend. Kevin Sheekey joined as campaign manager, along with longtime Bloomberg ad strategist Bill Knapp and top field organizer Mitch Stewart.

Bloomberg also named mayors Steve Benjamin of Columbia, South Carolina, and Greg Fischer of Louisville, Kentucky, as his first two co-chairs.