Now Playing: State of the Union fact-checking and analysis

Now Playing: State of the Union: 7 lines to remember

Now Playing: Impeachment talks heat up

Now Playing: Trump meets with Japanese prime minister

Now Playing: 1-on-1 with 2020 presidential candidate Pete Buttigieg

Now Playing: Trump attacking Pelosi helps 'unite her within that caucus even more': Matthew Dowd

Now Playing: Sending more troops to Middle East 'exactly the right thing' to do: Liz Cheney

Now Playing: Buttigieg: Would put my mayoral, military experience up 'against ... any competitor'

Now Playing: A federal judge is halting part of Trump's construction of the border wall

Now Playing: Pardoning war criminals undermines country's 'legal and moral' foundations: Buttigieg

Now Playing: Sanders holds first Vermont rally of 2020 cycle

Now Playing: Judge blocks Trump's administration from using military funds for the border wall

Now Playing: Theresa May resigns as British prime minister after months of pressure

Now Playing: Trump defends war of words with Pelosi: 'She made horrible statements'

Now Playing: Federal judge blocks law making most Mississippi abortions illegal

Now Playing: War of words between Trump and Pelosi

Now Playing: Millennials who won: Connecticut's youngest state senator

Now Playing: Nancy Pelosi is not helping this country': Trump

Now Playing: Rep. Jerry Nadler falls ill at NYC event

Now Playing: Pete Buttigieg takes on President Trump over Vietnam War medical deferment