By Veronica Stracqualursi, CNN

(CNN) -- Republican National Committee Chairwoman Ronna McDaniel sided with President Donald Trump over her uncle Mitt Romney, slamming the Utah senator-elect for writing an op-ed in which he criticized the President.

"For an incoming Republican freshman senator to attack (Donald Trump) as their first act feeds into what the Democrats and media want and is disappointing and unproductive," McDaniel wrote on Twitter Wednesday, choosing not to name her uncle. As of Wednesday morning, she had also "pinned" the tweet to the top of her profile.

Romney, a former Massachusetts governor and the 2012 Republican presidential nominee, will be sworn into the US Senate on Thursday.

He revived his feud with Trump by writing a scathing op-ed in The Washington Post published Tuesday, criticizing the President's behavior as "evidence that the President has not risen to the mantle of the office."

On Twitter Wednesday, Trump bristled at Romney's op-ed and told the incoming senator to "be a team player."

"I won big, and he didn't," Trump added in a knock to Romney's failed bid for president in 2012.

"Question will be, is he a Flake? I hope not," Trump wrote, referring to retiring Republican Sen. Jeff Flake, who is a frequent critic.

McDaniel has been supportive of Trump since the 2016 election and, as the former chairwoman of the Michigan Republican Party, helped Trump win the key state -- the first time a GOP presidential candidate had won Michigan in nearly 30 years.

She was tapped in December 2016 to succeed Reince Priebus as RNC chair while he took on the White House chief of staff job, from which he was later fired. McDaniel became only the second woman to chair the RNC in history.

Since then, McDaniel has had to navigate the tricky role of being a Trump supporter and Romney's niece. That could continue to prove difficult as her uncle begins his six-year term in Congress on Thursday. The incoming senator has made clear he will support the President where he agrees and speak out when he doesn't.

But she has taken steps to distance herself from her uncle. In 2017, The Washington Post reported that McDaniel obliged with Trump's request to drop the use of her maiden name in official party communications.

"It was just a difference of opinion as to where the country needed to go but it didn't affect my relationship with my Uncle Mitt," McDaniel told CNN's Dana Bash back in 2017 of Romney's 2016 speech where he slammed Trump as a "phony, a fraud."

Trump has asked McDaniel to stay on through the 2020 election.