Stzrok was immediately removed from special counsel Robert Mueller's team this past summer after the text messages were uncovered through an internal investigation at the Justice Department.
But when Rosenstein was asked by lawmakers if he had seen good cause to fire Mueller, whom he appointed and whose work he oversees, he replied that he had not.
"We recognize we have employees with political opinions".
"I think hes got a tough job to do, but it seems hes running far afield", said Sen.
Rosenstein also said he and Mueller talked about what his office was allowed to investigate and what it was not, though he declined to answer directly whether he had granted Mueller permission to expand his mandate.
Republicans in recent months have attacked Mueller, who has so far charged four Trump associates in connection with his investigation into whether Russian Federation meddled in the US presidential election a year ago and colluded with Trump's campaign.
Republicans on Capitol Hill, meanwhile, are asking if the Obama-led Department of Justice colluded with Democrats past year to take that questionable dossier to a federal court to obtain a warrant to spy on the Trump campaign. House Judiciary Committee Chairman Bob Goodlatte, R-Va., referred to the texts in his opening statement, saying they were "deeply troubling to all citizens who expect a system of blind and equal justice".
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Zebley previously served as Mueller's chief of staff at the Federal Bureau of Investigation and as a senior counselor in the National Security Division at the Department of Justice.
The officials who exchanged the messages - senior FBI agent Peter Strzok and senior FBI lawyer Lisa Page - once worked for Mueller's team and were key players in a prior investigation of former secretary of state Hillary Clinton's use of a private email server.
In July 2016, the two mock the Trump campaign and Republicans over their convention guests and speakers, including a Duck Dynasty star and Scott Baio.
Even before the messages were revealed, some Republicans had accused Mueller's team of harboring inappropriate bias, pointing to political contributions by several members to Clinton or other Democrats.
MSNBC's Rachel Maddow last night cites a "fire-Mueller freakout on the right": "Everyone's throwing red flags on the possibility of firing Robert Mueller, on both sides of the aisle".
The texts offered new ammunition. They also wrote messages disparaging House Speaker Paul Ryan, with Page expressing the hope that Ryan "fails and crashes in a blaze of glory" and Strzok responding that the GOP "needs to pull their head out of that *ss".
In one such back-and-forth from August 2015, Page texts: "I just saw my first Bernie Sanders bumper sticker. Figure they cancel each other out". He told Politico that he had warned Trump just days before, while in flight on Air Force One, that Muellers investigation was "infected with bias". Rosenstein said he was not. "Made me want to key the vehicle", Page wrote. "And there is no credible reason to diminish the outstanding and distinguished work done by the FBI over many years simply because the president wants to deflect or distract from the ongoing special investigation of himself".