WASHINGTON (AP) — As President Donald Trump considers a replacement for fired FBI Director James Comey, lawmakers are urging the president to steer clear of appointing any politicians.

The advice came Sunday amid more criticism over Trump's dismissal of Comey during an FBI probe of Russia's meddling with last year's election and any ties to the Trump campaign. James Clapper, the former director of national intelligence, said the Founding Fathers created three co-equal branches of government with checks and balances, but with Trump as president, that was now "eroding."

"I think, in many ways, our institutions are under assault, both externally — and that's the big news here, is the Russian interference in our election system," Clapper said "I think as well our institutions are under assault internally."

When asked, "Internally, from the president?" Clapper responded, "Exactly."

The White House had no immediate comment. No White House aide appeared on the Sunday news shows, leaving Nikki Haley, the U.S. ambassador to the United Nations, to defend Trump. "The president is the CEO of the country. He can hire and fire whoever he wants," she said.

Lawmakers from both parties reprimanded Trump for his actions, which included shifting explanations from the White House for Comey's dismissal and an ominous tweet by Trump that warned Comey against leaks to the press because Trump may have "tapes" of their conversations.

Republican Sen. Lindsey Graham, a member of the Judiciary Committee, said selecting an FBI agent to lead the agency would allow the nation to "reset." He dismissed as less desirable at least two of the 14 candidates under consideration, ex-FBI agent and former Rep. Mike Rogers of Michigan and Sen. John Cornyn of Texas, the No. 2 Republican in the Senate.

"It's now time to pick somebody who comes from within the ranks, or has such a reputation that has no political background at all that can go into the job on Day 1," said Graham, R-S.C.

"The president has a chance to clean up the mess he mostly created," Graham said, adding: "I have no evidence that the president colluded with the Russians at all ... but we don't know all the evidence yet."

Sen. Mike Lee, R-Utah, continued to argue that the president should consider Merrick Garland, the federal judge nominated to the Supreme Court last year by President Barack Obama but who was denied a hearing by Republicans. A former top aide to Sen. Mitch McConnell, Josh Holmes, said that McConnell is interested in the suggestion.

Senate Minority Leader Chuck Schumer of New York said the new FBI director should be someone "not of partisan background" with "great experience" and "courage." Declining to comment on a Garland nomination, Schumer left open the possibility that Democrats might withdraw support for a new FBI director unless the Justice Department names a special prosecutor for the Russia probe.

Under Senate rules, Republicans could confirm an FBI director with 51 votes. Republicans hold 52 seats in the chamber to Democrats' 48.

Calling Trump's remarks about possible taped conversations "outrageous," Sen. Mark Warner of Virginia, the top Democrat on the Senate Intelligence Committee, said his panel or another committee would "absolutely" subpoena such tapes.

"We have got to make sure that these tapes, if they exist, don't mysteriously disappear," Warner said, adding that he hopes to have Comey testify in a public hearing before his committee.

The blowback against the firing of Comey angered the increasingly frustrated president, who made the decision after consulting only a small group of advisers, worried the news would leak out. Trump has openly vented his frustration with the media and Democrats on Twitter, musing about canceling press briefings and arguing that it's difficult for aides to know his thinking.

The administration has interviewed at least eight candidates to replace Comey, just over half of the 14 being considered. Trump has said a decision could come before he leaves Friday for the Mideast and Europe, his first overseas trip as president. He was also set to welcome foreign leaders to the White House, with Turkey's President Recep Tayyip Erdogan on Tuesday and President Juan Manuel Santos of Colombia on Thursday. A leader of the United Arab Emirates was scheduled to visit Monday.

Clapper and Schumer made their comments on CNN's "State of the Union"; Graham spoke on NBC's "Meet the Press"; Haley and Warner appeared on ABC's "This Week"; and Warner spoke on "Fox News Sunday" along with Lee and Holmes.

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Associated Press writers Catherine Lucey, Sadie Gurman and Darlene Superville contributed to this report.