Washington (CNN) President Donald Trump intends to nominate acting Defense Secretary Patrick Shanahan to be his permanent pick to replace James Mattis at the Pentagon, according to a tweet from White House press secretary Sarah Sanders.

"Based upon his outstanding service to the Country and his demonstrated ability to lead, President Trump intends to nominate Patrick M. Shanahan to be the Secretary of Defense," Sanders said in a statement.

Shortly after the announcement, Shanahan issued a statement saying he was "honored" to be chosen as the Pentagon's permanent head. The former Boeing executive said in comments to reporters after the announcement that he had been informed of the decision earlier Thursday.

"If confirmed by the Senate, I will continue the aggressive implementation of our National Defense Strategy," Shanahan said in a statement. "I remain committed to modernizing the force so our remarkable Soldiers, Sailors, Airmen, and Marines have everything they need to keep our military lethal and our country safe."

Shanahan took over the job in an acting role following the December resignation of Mattis, who left the administration over a series of policy differences with Trump, particularly over the President's initial decision to withdraw US troops from Syria.

While Mattis had intended to stay on until the end of February in part to allow time for his successor to be named and confirmed, Trump, reportedly angry over the coverage of the' resignation, decided to cut the transition period short, thrusting Shanahan into the role on January 1.

Shanahan cleared a major hurdle blocking him from becoming Trump's nominee last month when the Department of Defense's inspector general cleared him of allegations he violated ethics agreements by promoting the interests of his former employer, Boeing.

The White House and Pentagon had both indicated Shanahan could not be nominated until the IG investigation concluded. One White House official pointed to the IG report clearing Shanahan as the main hurdle that he had needed to clear to be tapped for the job. The official conceded that West Wing aides had anticipated that Shanahan would be nominated sooner than Thursday, as the report came out weeks ago.

The top Democrat on the Senate Armed Services Committee, perhaps previewing criticism Shanahan can expect to face in the confirmation process, said the permanent role would be a "very different job than his previous position."

"Serving as secretary of defense is an immense task and I look forward to hearing directly from the acting secretary during the confirmation process," said Sen. Jack Reed of Rhode Island.

Sen. Jim Inhofe, an Oklahoma Republican who's the committee chairman, issued a statement saying he was pleased with the nomination.

"We need a confirmed leader at the department and, after working with him closely over the last few months, I welcome his selection," Inhofe said.

The announcement on Thursday came as the Trump administration faces foreign policy challenges on multiple fronts. On Wednesday, Iranian President Hassan Rouhani said Iran would reduce commitments to the joint nuclear agreement -- a year after Trump announced the US was withdrawing from the deal the Obama administration championed. The Trump administration has supported the efforts of Venezuelan opposition leader Juan Guaido in a standoff with the nation's president, Nicolas Maduro, and just hours before the announcement, South Korea said North Korea had launched two suspected "short-range missiles."

In a nod to the global flashpoints, Shanahan said the biggest challenge he would face was "balancing it all."

"As you can tell, there are real world events that happen every day, so you have to spin a lot of plates," Shanahan said at the Pentagon.

White House debate

CNN previously reported that Trump had told aides that he has been impressed by Shanahan's tenure as acting secretary and was inclined to nominate him to formally take over the permanent role, one source close to the White House told CNN in late February. When some of his advisers suggested other candidates, Trump responded by saying he is inclined to keep Shanahan.

A source close to the White House had said previously that Trump leaned toward nominating Shanahan to the post because he believed he would not push any particular policy agenda on the President -- a major source of tension between Trump and Mattis.

And while Mattis was largely seen as having the stature to push back against some of Trump's nontraditional policies and ideas, Shanahan is seen as much less likely to do so.

"This is the department of get stuff done," Shanahan has reportedly said on multiple occasions.

However, Thursday's announcement was the result of lobbying by White House acting chief of staff Mick Mulvaney as well.

In March, some West Wing officials were no longer convinced Shanahan would get the defense secretary job. Shanahan and other administration officials, including Mulvaney, repeatedly pressed Trump to nominate him, with Shanahan arguing that he could not act at his fullest capacity if he was just acting secretary.

But this pressure campaign annoyed Trump, sources said, and led him to begin quizzing people about who should become the permanent secretary.

"Fine, tell me who to nominate," Trump told one ally at the time.

Once they realized he was not set on Shanahan, outside forces began pitching names to Trump, like Army Secretary Mark Esper and former Air Force Secretary Heather Wilson, though people close to Trump doubted he would ever get on board with Wilson because she was dismissive of Space Force.

But Mulvaney succeeded in his efforts to get Shanahan nominated. He developed a liking for the former Boeing executive early on and advised the President he needed someone who was not a general.