Donald Trump has picked Neil Gorsuch as his Supreme Court nominee. Democrats have vowed to block the nomination after Republicans didn't even hold hearings on Obama's pick for the vacancy.
It used to take 60 votes in the Senate to confirm any presidential nomination. But after Republicans stymied many Obama picks, Democrats in 2013 changed the rules to let presidential appointees move forward with simple majorities (51 votes). Changing the rules like this was known as the "nuclear option."
Why it matters:
Supreme Court justices were excluded from the rule change. Mitch McConnell has said he doesn't want to expand the nuclear option to the court, but intense opposition to Trump's pick may force the hand of Republicans.