The Supreme Court today will hear arguments on President Donald Trump’s consequential first-day executive order in 2025, a case that could upend a long-held understanding on who is considered a U.S. citizen.

Can Trump change the definition of birthright citizenship with a stroke of his pen? At issue is whether babies born in the U.S. are automatically citizens.

Here's how President Trump's attempt to sharply limit birthright citizenship could affect millions of Americans.