WASHINGTON, Jan 6 (Reuters) - U.S. President Donald Trump said on Tuesday that Republicans must win the 2026 congressional midterm elections - or else he will get impeached by Democrats. "You gotta win the midterms 'cause, if we don't win the midterms, it's just gonna be - I mean, they'll find a reason to impeach me," Trump told Republican lawmakers at a retreat in Washington. "I'll get impeached." Ahead of the November elections, which could stall his agenda and expose him to congressional investigations, Trump teased and prodded allies who narrowly control the U.S. House of Representatives. He told them...