WASHINGTON — President Obama on Wednesday nominated Merrick B. Garland to be the nation’s 113th Supreme Court justice, choosing a centrist appellate judge who could reshape the court for a generation and become the face of a bitter election-year confirmation struggle.

In selecting Judge Garland, 63, a well-known figure in Washington legal circles who has drawn praise from members of both parties, Mr. Obama dared Republican senators to ignore public pressure and make good on their promise to block consideration of any nominee until after the next president is chosen.

The nomination to fill the seat on the court created by the death of Justice Antonin Scalia sets in motion a standoff that is likely to play out for many months, perhaps without resolution. Its outcome could tip the ideological balance of the nation’s highest court, and an array of well-financed interest groups on both sides has already assembled for the battle over the nomination.