Washington (CNN) President Barack Obama is looking for a "consensus candidate" to nominate to the Supreme Court who could gain broad support from Democrats and Republicans, he said in an interview airing Monday on CNN.

"It is my intention to nominate somebody who has impeccable credentials, somebody who should be a consensus candidate, is deserving to be on the Supreme Court, and I will continue to challenge the Republicans in the Senate who suggest somehow that they don't have to do their job in providing that the nominee hearing and a vote," Obama told CNN en Español anchor Juan Carlos Lopez.

"I'm moving forward on interviewing candidates, and I will be making a determination soon," Obama said in the interview taped last week. While interviews for potential candidates were thought to be underway, Obama and his aides had previously refused to publicly comment.

Liberal groups have been closely communicating with the White House on strategy, including during meetings and conference calls that began the week after Justice Antonin Scalia died in mid-February.

Senate Republicans, meanwhile, have said they will not hold a hearing with the potential high court nominee, wishing to leave the decision to the next president. And the Republican National Committee is preparing to fight the pick as well.

Obama's announcement is expected as early as this week, pending final vetting on a small list of candidates for the high court. Sources have told CNN that three sitting federal appellate judges top the list: Sri Srinivasan and Merrick Garland of the U.S. Court of Appeals for the District of Colombia Circuit, and Paul Watford, who serves on the 9th U.S. Circuit Court of Appeals in California.

All three would fit Obama's description of a "consensus candidate" that previously have gained support from Republican senators, though Srinivasan was confirmed the most recently, in 2013, and with the broadest support from the GOP (he was confirmed unanimously).

In previous interviews and remarks, Obama hasn't said specifically that he's seeking a candidate with Republican appeal. At a news conference three days after Scalia's death, Obama told reporters that he wasn't necessarily going to name a moderate nominee. In multiple settings, Obama has said only that he's seeking candidates who have solid records, a respect for the court's role, and an ability to bring real-life experience to the bench.

Others in the administration, including Vice President Joe Biden, have pushed for a nominee that's previously enjoyed Republican support.

Speaking Monday, White House press secretary Josh Earnest indicated that Obama continued to review background material on potential candidates the weekend, but wouldn't give an indication of whether he's narrowed his shortlist.

"There's ample time for the President to make a decision and for the Senate to fulfill its constitutional responsibility to offer its advice and consent and still have the President's nominee seated on the Supreme Court before the next term starts," Earnest said.

White House outreach

The White House's outreach is coordinated by former White House staffer Stephanie Cutter, who has led sessions with grassroots groups designed to cement a strategy once a nominee is named. The beginnings of a pressure campaign have centered on select Republican lawmakers: Sen. Chuck Grassley, the Republican chairman of the Judiciary Committee, who has steadfastly refused to budge considering Obama's eventual nominee, along with a group of vulnerable Republicans up for reelection in November.

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A source involved in the effort said the pressure campaign is designed to force the GOP to crack by the end of June -- before the party's nominating conventions and the August recess.

"The coordinated grassroots effort that has already proven a powerful tool to put pressure on Republicans will only ramp up," a former White House communications official who is helping coordinate the public roll-out of the President's nominee. "We will be mobilizing our network of allies in the states to promote the nominee, and to continue to pressure Republicans on their position of obstruction. That includes events in targeted states with real working Americans pushing Senate Republicans to do their jobs, press events with key Democratic members and groups, and coordinated validator pushes like those with the legal scholars, historians, law deans and attorneys general."

Obama's disclosure that he is seeking a "consensus candidate" fits into that strategy. If enough Republicans have previously supported Obama's pick for a lower court, the White House hopes to cast them as politically driven if they refuse to consider the name for the Supreme Court.

Liberal groups have already begun those efforts, including organizing rallies in senators' home states to advocate hearings for Obama's eventual nominee. Democratic activists rallied at courthouses in Iowa this weekend to urge Grassley, who represents the state, to reconsider his position staunchly opposing hearings for the Obama pick.

Difficulty getting Republican cooperation

But the White House's effort to find Republican lawyers willing to offer testimonials on behalf of a potential high court nominee has run into some hurdles, according to people close to the process. Some former clerks and legal colleagues who know the finalists have balked at the prospect of becoming part of a partisan fight over the nomination, they said.

The testimonials are part of the communications strategy of every nomination process, and typically White House officials and outside consultants providing assistance help prepare bipartisan voices to help bolster the case for a nominee.

In recent days, officials helping organize the testimonials have reached out to people who are known to have worked with potential nominees and who have Republican affiliations. The negative response from some shows the pressure building on both sides of the political aisle even before the president announces his pick.

A White House spokesman declined to comment on this point.

And Republicans are planning a coordinated effort to discredit Obama's nominee, led by the party's national committee. RNC Chairman Reince Priebus said his "SCOTUS task force" would amount to "the most comprehensive judicial response effort in our party's history."

The strategy includes media appearances and opposition research on potential selections, as well as highlighting past statements from Democrats demonstrating support for withholding Supreme Court nominations in election years.

One conservative group, the Judicial Crisis Network, said it was spending six figures to lambast a potential nominee, Jane Kelly, in markets around the country. The ad hits the former public defender, who now serves on the federal bench, for representing a child molester.

Targeting Grassley

Democrats pushed back sharply on the depiction, calling on Grassley, the Judiciary panel chairman, and other Republicans to denounce the Kelly ad. Kelly serves on the 8th U.S. Circuit Court of Appeals and is based in Iowa.

"It is an adult moment for Grassley to set the tone going forward that these kinds of tactics by his friends on the right are not welcome in this process -- a process that should be free of these type of political smears," said Brad Woodhouse, the president of the liberal group Americans United for Change.

Grassley will be a primary focus of Democratic pressure.

A Democrat challenging Grassley for his Senate seat, Patty Judge, has vowed to keep the issue at the forefront in her campaign against him. In other races around the country, including in New Hampshire and Alaska, the issue has similarly entered the campaign trail rhetoric. Activist groups are planning events in senators' home states during next week's congressional recess.

Grassley has shown little indication of backing down, facing equal pressure from conservatives to maintain his position. A person who met with Grassley recently said he pounded the table at one point to emphasize that he won't reconsider his stance.

The Democrats' effort will include leveraging law professors and experts who argue against allowing a seat to remain vacant on the Supreme Court for months. The latest came from Patricia Wald and John Gibbons, former chief judges of federal appeals court, who wrote in a letter to Grassley and Senate Majority Leader Mitch McConnell that a full component of judges "is essential to the Court's primary function of declaring what the law is in a rapidly moving society where crises frequently arise."

Depending on the outcome of the Republican primary, Obama's allies will also point to a potential Donald Trump nominee as the alternative -- a prospect that Republican senators, some of whom haven't even said they'll support Trump if he's the nominee, are loathe to embrace.