(Bloomberg) — Fresh off his summit with North Korean leader Kim Jong Un, Donald Trump returns to Washington on Wednesday to face what may be the biggest threat to his presidency — Special Counsel Robert Mueller.

Now, Mueller is intent on quickly resolving a central issue with Trump’s legal team: whether the president will sit voluntarily for an interview in the probe of Russian election meddling, according to current and former U.S. officials. After months of negotiations, the two sides must find common ground or gear up for an unprecedented legal fight likely to go all the way to the Supreme Court.