WASHINGTON — President Barack Obama said today he will reveal his Supreme Court nominee to fill the vacancy of the late Justice Antonin Scalia, and his pick is expected to come from a small circle of federal appeals court judges.

“I’ve made my decision,” Obama said in an email to supporters early this morning.

Obama planned to introduce his pick at 11 a.m. in the White House Rose Garden, setting up a showdown with Senate Republicans who have told the White House not to bother filling the vacancy in an election year.

In his email, Obama did not identify his choice to replace the Scalia on the nine-member court. But the president said he had devoted a “considerable amount of time and deliberation to this decision” and consulted with outside experts and groups.

The Associated Press has reported that Obama had narrowed the list to three appeals court judges: Merrick Garland, the chief judge of the appeals courts in Washington, D.C.; Sri Srinivasan, a judge on that court; and Paul Watford of the appeals courts based in San Francisco.

“In putting forward a nominee today, I am fulfilling my constitutional duty. I’m doing my job,” Obama wrote. “I hope that our senators will do their jobs, and move quickly to consider my nominee.”

That will be a hard sell because Republicans control the Senate, which must confirm any nominee, and GOP leaders want to leave the choice to the next president, denying Obama a chance to alter the ideological balance of the court before he leaves office next January. Republicans contend that a confirmation fight in an election year would be too politicized.

Ahead of Obama’s announcement, the Republican Party set up a task force that will orchestrate attack ads, petitions and media outreach. The aim is to bolster Senate Republicans’ strategy of denying consideration of Obama’s nominee. The party’s chairman, Reince Priebus, described it as the GOP’s most comprehensive judicial response effort ever.

On the other side, Obama allies have been drafted to run a Democratic effort that will involve liberal groups that hope an Obama nominee could pull the high court’s ideological balance to the left. The effort would target states where activists believe Republicans will feel political heat for opposing hearings once Obama announced his nominee.

In his email, Obama said his nominee will be “eminently qualified” to sit on the nation’s highest court. He said the nominee would understand the limits of the judiciary’s role and “grasps the way it affects the daily reality of people’s lives in a big, complicated democracy, and in rapidly changing times.”

Obama said the White House had “reached out to every member of the Senate, who each have a responsibility to do their job and take this nomination just as seriously.”

The president told supporters his nominee “deserves a fair hearing, and an up-or-down vote.”