President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE attacked Pennsylvania Rep. Conor Lamb (D) in a pair of tweets Saturday morning that endorsed Lamb's opponent, Rep. Keith Rothfus Keith James RothfusCNN's Tapper tried to talk GOP candidate out of running against Democratic incumbent: report Lobbying world Conor Lamb gets 2020 challenger touted by Trump MORE (R-Pa.), in the battle for the state's newly drawn 17th District.

Trump tweeted his support for Rothfus on Saturday as Vice President Pence headed to the state to campaign for Rothfus. He also resurfaced a nickname for Lamb, "Lamb the Sham."

ADVERTISEMENT

The two congressmen are dueling for the newly drawn district, currently held by Rep. Matt Cartwright Matthew (Matt) Alton CartwrightRaces heat up for House leadership posts Trump Jr. seeks to elect 'new blood' to Republican Party Republicans face worsening outlook in battle for House MORE (D), who himself was redistricted to Pennsylvania's 8th District.

".@VP Pence is heading to Pittsburgh, Pennsylvania where we have both strongly endorsed one of the finest men around, Congressman Keith @RothfusForPA. He is running against #LambTheSham, who is telling everyone how much he likes me, but he will only vote with Nancy Pelosi Nancy PelosiHoyer: House should vote on COVID-19 aid — with or without a bipartisan deal Ruth Bader Ginsburg lies in repose at Supreme Court McCarthy threatens motion to oust Pelosi if she moves forward with impeachment MORE. Keith is strong on borders and tough on crime — and loves cutting taxes! #MAGA" Trump tweeted.

.@VP Pence is heading to Pittsburgh, Pennsylvania where we have both strongly endorsed one of the finest men around, Congressman Keith @RothfusForPA. He is running against #LambTheSham, who is telling... — Donald J. Trump (@realDonaldTrump) June 23, 2018

...everyone how much he likes me, but he will only vote with Nancy Pelosi. Keith is strong on borders and tough on crime — and loves cutting taxes! #MAGA — Donald J. Trump (@realDonaldTrump) June 23, 2018

Trump has previously claimed that Lamb was elected to Pennsylvania's 18th District in a special election this year only because of his support for the Trump administration's policies, including erroneously stating that Lamb supported the GOP tax plan.

“The young man last night that ran, he said, ‘Oh, I’m like Trump. Second Amendment, everything. I love the tax cuts, everything.’ He ran on that basis,” the president told donors after Lamb's victory, according to The Atlantic. “He ran on a campaign that said very nice things about me. I said, ‘Is he a Republican? He sounds like a Republican to me.’ ”

The Cook Political Report rates Pennsylvania's newly drawn 17th District, the result of the state Supreme Court finding the old state map unconstitutional, a "toss-up" heading into November's midterm elections where Democrats hope to pick up 23 seats and retake the House.

This article was updated at 11:33 a.m. to clarify Rep. Lamb's stance on the GOP tax reform bill, passed in December.