Dave McCormick, who privately lobbied hard to win former President Trump’s blessing, is going after Trump for backing Dr. Oz in the country's hottest current race — the Pennsylvania GOP race for U.S. Senate.

Why it matters: Trump slammed McCormick Friday night at a rally in Pennsylvania. Now McCormick is punching back in an ad you're seeing first on Axios — ridiculing the endorsement of Oz.

McCormick's campaign tells Axios the ad will begin airing on TV on Monday as the main ad for the week — the last week before the primary on May 17.

• The ad, called "Wrong Endorsement in Pennsylvania," includes a clip of Fox News' Laura Ingraham saying on her show: "It was a mistake to endorse Oz."

The big picture: Trump attacked McCormick for the first time Friday, at a rainy, muddy rally last night outside Pittsburgh.

• The former President said that as CEO of Bridgewater, the world's largest hedge fund, McCormick "managed money for communist China.”
• Trump asserted McCormick is the "candidate of special interests and globalists and the Washington establishment ... ripping off the United States with bad trade deals and open borders," AP writes.

Trump spent more time criticizing McCormick than he did praising Oz, the Pittsburgh Post-Gazette reports.

• McCormick "may be a nice guy, but he’s not MAGA," Trump said.
• The crowd booed when a warm-up speaker mentioned Oz.

Context: McCormick wanted Trump's backing badly, turning to his wife, Dina Powell, a former Trump official, and Trump-friendly advisers like Hope Hicks to plead his case. They mostly pushed Trump to remain neutral in the race, but he stiffed McCormick in a very public way.

• A poll released this week by Franklin & Marshall College in Lancaster, Pa., showed a statistical tie — Oz 18% and McCormick 16% among Republicans, with conservative activist Kathy Barnette at 12%.
• McCormick is backed by Mike Pompeo, Trump's former Secretary of State.

Reality check: The ad calls Oz "PRO-ABORTION." Oz formerly spoke in favor of abortion rights, but now says he is "100% Pro-Life."

McCormick said this morning on "Fox & Friends," from a diner in Sewickley, Pennsylvania:

• "President Trump's very popular in Pennsylvania — and with good reason, because his America First policies absolutely were great for Pennsylvanians and for Americans. The problem is Mehmet Oz isn't popular in Pennsylvania."