Donald Trump Jr., son of President Trump, speaks to the media as he departs a closed meeting with members of the Senate Intelligence Committee on Wednesday. (Zach Gibson/EPA-EFE/Shutterstock)

President Trump’s eldest son, Donald Trump Jr., signaled Thursday that he plans to campaign for a primary challenger to Justin Amash (Mich.), the sole Republican congressman who has called for the president’s impeachment.

“See you soon Justin,” Trump Jr. said in a morning tweet. “I hear Michigan is beautiful during primary season.”

Attached to his tweet were poll results showing Amash trailing Jim Lower, a state lawmaker who announced last month that he would mount a primary challenge to Amash in Michigan’s 3rd Congressional District.

Trump Jr.’s tweet served to fuel speculation that the president would also become actively involved in trying to topple Amash.

Amash became a pariah among fellow House Republicans after declaring last month that President Trump had engaged in “impeachable conduct” that was detailed in the report of now-former special counsel Robert S. Mueller III.

Roughly 60 Democrats have called for launching impeachment proceedings against Trump, with most pointing to episodes of possible obstruction of Mueller’s investigation into Russian election interference in 2016.

At a recent town hall meeting in Grand Rapids, Amash said Congress “has a duty to keep the president in check.”

“I’d do it whether it was a Republican president or a Democratic president,” he said. “It doesn’t matter. You elected me to represent all of you.”

Amash responded later Thursday to Trump Jr.’s vow to visit during primary season, writing on Twitter: “if it’s what you say I love it especially later in the summer.”

That was a reference to Trump Jr.’s role in setting up a meeting in Trump Tower with Russians purportedly to receive damaging information about Hillary Clinton.

Responding to an email sent by an intermediary promising “sensitive information” about Clinton, Trump Jr. wrote: “If it’s what you say I love it especially later in the summer.”

Lower is openly courting the support of the president.

During a television appearance Wednesday night, he told Fox News host Laura Ingraham that he would like Trump’s endorsement and hoped that he was watching..

“I’d like to see the president get directly involved,” Lower said.

Lower also touted his support for Trump and accused Amash of having “basically left the Republican Party.”

He cited Amash’s vote Wednesday to hold Attorney General William P. Barr and Commerce Secretary Wilbur Ross in contempt of Congress over the administration’s efforts to shield documents related to its decision to add a citizenship question to the 2020 Census.