Visitors view former US President Bill Clinton's portrait during the exhibition "America's Presidents" at the National Portrait Gallery in Washington D.C., the United States, on Feb. 17, 2020. (Xinhua/Liu Jie)

Visitors view former US President Barack Obama's portrait during the exhibition "America's Presidents" at the National Portrait Gallery in Washington D.C., the United States, on Feb. 17, 2020. (Xinhua/Liu Jie)

Visitors view former US President George W. Bush's portrait during the exhibition "America's Presidents" at the National Portrait Gallery in Washington D.C., the United States, on Feb. 17, 2020. (Xinhua/Liu Jie)

Visitors view former US President Abraham Lincoln's portrait during the exhibition "America's Presidents" at the National Portrait Gallery in Washington D.C., the United States, on Feb. 17, 2020. (Xinhua/Liu Jie)