Trump said later on Tuesday that he did not ask Shanahan to withdraw from the nomination process, according to Reuters.
US-based media outlets reported domestic violence allegations against Shanahan in 2010. Trump told reporters, cited by Reuters, that he heard about the allegations for the first time on Monday, adding that his administration had a good vetting process for potential nominees.
Senator Richard Blumenthal (D-CT) said that the Pentagon's internal watchdog should investigate whether the White House concealed information about Shanahan's past that could have disqualified him for the post.
“There is absolutely no reason for the failure to disclose to the [Senate] Armed Services Committee as a part of our [confirmation] process that there was something in this nominee’s past, Patrick Shanahan’s past”, Blumenthal told reporters. “There ought to be an investigation of that whole process".
Blumenthal added that the White House may have committed a "violation of criminal law" by lying to the Armed Services Committee. He called for the Defence Department's Inspector General to launch an investigation into the matter.
On Monday, Shanahan announced that the US would send an additional 1,000 soldiers to the Middle East to address what Washington claimed were air, naval and ground-based threats in the region. The Pentagon has been stepping up the US military presence in the Middle East since May, in what National Security Adviser John Bolton described as a clear message to Iran.
Relations between the US and Iran have become increasingly hostile since 2018, after the Trump administration unilaterally withdrew from the 2015 nuclear agreement with Iran and began re-imposing sanctions on the Islamic Republic.
Senate Majority Leader Mitch McConnell (R-KY) told reporters on Tuesday that the US would be better off in its confrontation with Iran to have a secretary of defense who has been confirmed by the Senate rather than a temporary acting official, as is currently the case.
Trump said on Tuesday that the US is prepared with regard to Iran.
In recent months, senior US officials, including Secretary of State Mike Pompeo, National Security Adviser John Bolton and acting Pentagon chief Patrick Shanahan have been reportedly making the case that the Trump administration has the legal authority to go to war against Iran based on the 2001 Authorization for Use of Military Force (AUMF), Politico reported.
The two-decade-old law authorizes the use of military force against those behind the 11 September 2001 terrorist attacks in the US.
The legislation was originally intended to authorize the US war in Afghanistan, but successive administrations have cited the AUMF to justify various military actions around the world, including in Iraq, Somalia, Syria, Georgia, and Yemen.
Politico reported, citing a senior administration official, that the White House still hopes that its maximum-pressure policy will force Iran back to the negotiating table to discuss a new nuclear deal.