WASHINGTON (Reuters) - The American Bar Association issued a glowing evaluation of President Barack Obama's U.S. Supreme Court nominee on Tuesday, giving Merrick Garland its highest rating even as Senate Republicans stood firm in their blockade of his appointment.

The influential lawyers association, which has evaluated the qualifications of nominees to the federal judiciary since 1953, deemed Garland "well qualified," the highest of its three ratings, for a lifetime job on America's highest court.

"The unanimous consensus of everyone we interviewed was that Judge Garland is superbly competent to serve on the United States Supreme Court," the ABA panel that conducted the evaluation said in a report.

Obama nominated Garland, 63, on March 16 to replace conservative Justice Antonin Scalia, who died on Feb. 13. Garland is a centrist jurist with two decades as a federal appeals court judge.

A nominee's ABA rating typically is considered when the Senate acts on a judicial appointment.

Republicans were unswayed. In an emailed statement, Senate Majority Leader Mitch McConnell's office pointed out that Obama and Vice President Joe Biden, when they were senators, tried to block the 2006 Supreme Court confirmation of conservative Samuel Alito, even though he also earned the ABA's highest rating.

Supreme Court nominations require Senate confirmation, and the Republicans who control the chamber have denied Garland customary hearings and a confirmation vote. Senate Republican leaders have vowed not to act on any Supreme Court nominee put forward by Obama, arguing that the next president, to be elected on Nov. 8 and take office on Jan. 20, should fill Scalia's vacancy.

Obama has accused Senate Republicans of abdicating their constitutional duties. The court is now split 4-4 between conservatives and liberals, meaning Scalia's successor could influence its ideological direction for years to come.

The White House renewed its demand that the Senate act on Garland. Spokesman Josh Earnest said "it is such a shame he has been treated so unfairly by the United States Senate" in an "unprecedented partisan action."

The Republicans are hoping their presumptive nominee, Donald Trump, wins the presidency and nominates a conservative to the court.

"This significant point warrants repeating," the ABA panel wrote.

"All of the experienced, dedicated and knowledgeable sitting judges, several former solicitor generals from both political parties, legal scholars from top law schools across the country, and lawyers who have worked with or against the nominee in private practice, government or within the judiciary describe the nominee as outstanding in all respects and cite specific evidence in support of that view."

(Additional reporting by Richard Cowan; Editing by Will Dunham)