President-elect Trump’s decision to tap Rep. Matt Gaetz (R-Fla.) to become attorney general caught Senate Republicans completely flat-footed, and put the party on the defensive immediately over one of the most prominent — and polarizing — Cabinet selections.  Two key centrist members — Sens. Susan Collins (R-Maine) and Lisa Murkowski (R-Alaska) — expressed shock and surprise, with the Alaska Republican panning the choice for its lack of seriousness.  “I don’t think it’s a serious nomination for attorney general. We need to have a serious attorney general,” Murkowski told reporters. “I’m looking forward to the opportunity to consider somebody that is serious.” “This one was not on my bingo card,” she added.  Ever since Trump’s electoral romp last week, Republicans have been calling for a swift confirmation of his nominees to the Cabinet.  But some members slammed the brakes on that talk after the nomination of Gaetz, known as a prominent rabblerouser who orchestrated the ouster of former Speaker Kevin McCarthy (R-Calif.), to become the nation’s top cop.  Sen. Todd Young (R-Ind.), another potential swing vote, steadfastly refused to discuss Gaetz’s nomination, instead deciding to focus on Sen. Marco Rubio’s (R-Fla.) nomination roughly an hour earlier to become secretary of State.  “I’ll look at the others later. I don’t know a lot about them,” he said.  Among the main concerns for some members of the Senate Judiciary Committee and senators writ large will be the Department of Justice’s years-long investigation into allegations that he had sex with an underage girl. The investigation was closed last year and Gaetz was not criminally charged. He has denied any wrongdoing, but a House Ethics Committee probe is still ongoing.  Members are already bracing for that subject to come up in their role in considering Gaetz.  “Most certainly, it would be concerning because it’s part of his file,” said Sen. Mike Rounds (R-S.D.). “We normally give the president the benefit of the doubt, but we still do our due diligence and advice and consent is still important,” he also noted.  The news, coupled with the announcement only minutes earlier that he nominated former Rep. Tulsi Gabbard (Hawaii) to become the Director of National Intelligence, jarred Republicans from across the spectrum. The news came toward the tail-end of what was already an action-filled day that included Trump’s visit to Washington and a selection of a new Senate GOP leader. "[Trump] goes from the light hearted meeting with House Republicans to a love fest with Jill and Joe at the White House. It's this wonderful normal thing --- and then he essentially gets on the plane and nukes Washington,” one Senate Republican aide told The Hill, summing up the afternoon. But it was the Gaetz development that really hit hard for some members.  Sen. John Cornyn (R-Texas), fresh off his defeat in his run for leader, flashed a surprised expression across his face like he just bit into a sour lemon when reporters asked him to react to Gaetz’s nomination. The senior member of the Senate Judiciary Committee looked so flummoxed that it prompted a wave of laughter from the gaggle of reporters surrounding him. But the veteran lawmaker maintained message discipline, telling reporters: “We'll do our job and vet the nominee."  Asked if Gaetz would be his pick to serve as attorney general, Cornyn replied: "I don't get to choose, that's the prerogative of the president.” "Our prerogative is to do our job under the Constitution of advice and consent, which we will certainly do," he said.  Sen. Thom Tillis (R-N.C.), another member of the Judiciary Committee, said just because Gaetz is nominated doesn't mean he'll get the votes to be confirmed, predicting that not a single Democrat will vote for him.  “It'll just be interesting to see what his organic base is,” Tillis said of the nominee. "At the end of the day, Congressman Gaetz, he'll have a hearing but I'm all about counting votes and I would think that he's probably got some work cut out for him to get a good strong vote. “We're not going to get a single Democrat vote,” he said.  Trump and his allies can likely only afford to lose three votes on any nomination, assuming Republican David McCormick ends up winning his race in Pennsylvania against Sen. Bob Casey (D-Pa.). He leads by 28,000 votes, but the race is in the zone of an automatic recount.  Despite the margin, it still may be difficult for some Trump nominees, especially Gaetz and Gabbard to get across the finish line.  When asked if Trump was testing members, Rounds responded in the affirmative.  “It remains to be seen,” Rounds said with a hearty laugh. “But we have wondered whether that may be part of the discussion.”