Trump vs. Democratic mayors

• “Donald Trump, through his attacks, doesn’t realize that he is actually probably in some ways elevating the profile of these elected officials who work tirelessly in obscurity,” Ken Martin, chairman of the Minnesota Democratic-Farmer-Labor Party told Politico's Quint Forgey.
• “And now these elected officials … have developed a huge following and have become rising stars in the party.”

Catch up quick: Trump called Frey out on Twitter for requesting reimbursement for the $530,000 costs associated with security for a Trump rally taking place last Thursday. Frey fired back, and the city and Trump campaign are still in a "standoff" over the bill, he wrote in a weekend op-ed.

Other liberal mayors have drawn Trump's ire in the past, including San Juan Mayor Carmen Yulín Cruz and Dayton Mayor Nan Whaley.

• Several have forcefully defended their cities when they've been targeted in tweets, such as San Francisco Mayor London Breed and Baltimore Mayor Bernard "Jack" Young.

Frey offered some advice for mayors who suddenly find themselves in the cross-hairs: handle it "with civility and a bit of humor."

What's next: Expect more Twitter feuds between Trump and mayors over the next year. The sparring riles up voters in important districts, where Democratic mayors serve as convenient contrasts for Trump's stances.