President Obama on Wednesday said he would nominate Merrick B. Garland as the nation’s 113th Supreme Court justice, choosing a centrist appeals court judge for the lifetime appointment and daring Republican senators to refuse consideration of a jurist who is highly regarded throughout Washington. Mr. Obama introduced Judge Garland to an audience of his family members, activists, and White House staff in the Rose Garden Wednesday morning, describing him as exceptionally qualified to serve on the Supreme Court in the seat vacated by the death of Justice Antonin Scalia, who died in February.