WASHINGTON — The Senate is bracing for a tax-cut showdown that is all about Democrats and Republicans showing voters their differences over taxing the well-off while accusing each other of threatening to shove the government over a fiscal cliff.

Senators planned to vote Wednesday on a $250 billion Democratic bill that would extend expiring tax cuts next year for all but the highest earners. Democrats will need 60 votes to advance the proposal, which they do not have.

It seemed unlikely that senators also would vote on a rival GOP plan that includes the best-off Americans in the tax reductions, a measure that was destined to lose.