WASHINGTON (Reuters) - President Barack Obama said on Wednesday he hoped “cooler heads will prevail” and that the Republican-led Senate will act on his U.S. Supreme Court nominee but top Republicans dug in their heels, defending their refusal to consider anyone Obama picks. Obama has narrowed to five his list of candidates to replace conservative Justice Antonin Scalia, who died on Feb. 13. Obama’s nominee could tip the nine-member court to the left for the first time in decades. The Republicans who control the Senate have vowed not to hold confirmation hearings or an up-or-down vote on anyone Obama picks, saying the choice should belong to the next president who takes office in January after the Nov. 8 presidential election. “My hope is that cooler heads will prevail and people will reflect on what’s at stake here once a nomination is made,” Obama said at a news conference with Canadian Prime Minister Justin Trudeau. The White House is interviewing five candidates, federal judges Sri Srinivasan, Jane Kelly, Ketanji Brown Jackson, Paul Watford and Merrick Garland, according to a source familiar with the process. Senate Judiciary Committee Chairman Chuck Grassley, whose panel handles Supreme Court nominations, offered a lengthy defense of the Senate Republicans’ stance. Grassley accused Democrats of a “charade” with feigned outrage over the Republican refusal to consider Obama’s nominee simply to “score as many political points as possible.” “Regardless of what some are willing to admit publicly, everybody knows any nominee submitted in the middle of this presidential campaign isn’t getting confirmed. Everybody. The White House knows it. Senate Democrats know it. Republicans know it. Even the press knows it,” Grassley told a committee hearing. Under the U.S. Constitution, the president selects a Supreme Court nominee and the Senate confirms or rejects the nominee. “I’m going to do my job,” Obama said, promising an “eminently qualified” nominee. “And it will then be up to Senate Republicans to decide whether they want to follow the Constitution and abide by the rules of fair play that ultimately undergird our democracy and that ensure that the Supreme Court does not just become one more extension of our polarized politics,” Obama said.  Denis McDonough, Obama’s chief of staff, and other presidential aides met with Judiciary Committee Democrats at the White House on the nomination. Afterward, the Democratic senators predicted Republicans would buckle under public pressure and drop their “obstruction” once Obama names his nominee. “We are optimistic that, soon enough, not only will the president nominate, but our Republican colleagues will see the light,” said U.S. Senator Chuck Schumer, forecasting that Obama’s nominee will be confirmed with bipartisan support. Separately, one Republican senator indicated Senate Republicans would act on a nominee if they had a Republican president. “If a conservative president’s replacing a conservative justice, there’s a little more accommodation to it,” Wisconsin’s Ron Johnson told a radio interviewer. “President Obama’s nominee would flip the court from a 5-4 conservative to a 5-4 liberal-controlled court. And that’s the concern,” Johnson added. THE GRASSLEY-KELLY CONNECTION Grassley in 2013 spoke in favor of Kelly’s nomination to the St. Louis-based U.S. Court of Appeals for the 8th Circuit. She was confirmed by a 96-0 Senate vote. Kelly, an Obama classmate at Harvard Law School, is based in Iowa and previously served as a federal public defender there. The Iowa senator rejected any notion he could be persuaded to drop his opposition if Obama were to nominate a candidate previously confirmed by him and other Republicans. He denounced the idea that the White House selection process was “guided by the raw political calculation of what they think will exert the most political pressure on me.” Choosing someone like Kelly from Iowa would be an “obvious political ploy” that would fail, Grassley added. Senator Orrin Hatch, another Judiciary Committee Republican, said in an interview that Garland, whose previous nomination to the appellate court he backed, is “a fine man” who would be “a moderate choice” for the high court.  But Hatch said he opposed acting even on Garland. “It isn’t a question about the person in my opinion. It’s a question about the timing ... and the atmosphere that we have around here, which is poisonous,” Hatch said. Senate Democratic Leader Harry Reid delivered his latest attack on Grassley, saying on the Senate floor that it was “a little strange, a little odd” that Grassley would not hold hearings even for Kelly, considering his past support. Iowa’s Tom Miller, a Democrat, was among a group of attorneys general from 19 states, the District of Columbia and Puerto Rico who sent a letter to Grassley and other Senate leaders urging them to act promptly on Obama’s nominee. 