Sen. Jeff Merkley Jeffrey (Jeff) Alan MerkleyThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Sunday shows - Trump team defends coronavirus response Oregon senator says Trump's blame on 'forest management' for wildfires is 'just a big and devastating lie' MORE (D-Ore.) compared President-elect Donald 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 to a child on Thursday amid a furor over Trump's tweets about U.S. nuclear arms policy.

“Right now we have an incoming president who has kind of the maturity of a 5-year-old wrapped by massive ego. To have that just a second away from a nuclear trigger is very, very scary,” Merkley told MSNBC’s Chris Hayes on “All In” Thursday.

Referring to the intricacies of nuclear policy, he added, “This is an area where it’s constrained by international agreements, as it should be because of the risk of nuclear weapons,”

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“[Trump’s] going very much against the structure that his good friend[s] [former President Richard] Nixon and [former President Ronald] Reagan previously followed,” added Merkley.

Trump on Thursday tweeted that the U.S. “must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes.”

The president-elect’s comments startled the arms-control community, with some nuclear nonproliferation activists worrying it could start a global weapons race.

Trump spokesman Jason Miller on Thursday clarified the president-elect’s remarks in a follow-up statement.

“[Trump] was referring to the threat of nuclear proliferation and the critical need to prevent it — particularly to and among terrorist organizations and unstable and rogue regimes. He has also emphasized the need to improve and modernize our deterrent capability as a vital way to pursue peace through strength."

However, on MSNBC's "Morning Joe" on Friday, co-host Mika Brzezinski said she had spoken to Trump, seeking clarity on the issue, and he had responded, "Let it be an arms race. We will out-match them at every pass and outlast them all."