Comey's firing shows a 'deeply insecure president,' Sen. Tim Kaine says of Trump

Trump "understands that the noose is tightening," Kaine said.

Kaine said Trump's letter Tuesday to Comey "is very telling," adding that he believes the termination "is an attempt to obstruct the investigation into ties between Russia and the Trump campaign."

Trump's missive cited recommendations of the United States attorney general and the deputy attorney general, saying the termination was "effective immediately."

Comey's dismissal came just weeks after he testified in front of the House Committee on Intelligence, during which he took the rare step of confirming the FBI was investigating alleged Russian interference in the 2016 U.S. presidential election and "any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia's efforts."

Kaine said this morning he believes Comey's firing is part of "a pattern," which the former Democratic nominee for vice president said includes former acting U.S. Attorney General Sally Yates, former national security adviser Michael Flynn and U.S. Attorney General Jeff Sessions.

On Jan. 30, Trump replaced Yates, an appointee of former President Obama, as acting attorney general after she issued instructions to the U.S. Department of Justice not to defend his first travel ban executive order.

On Monday, Yates testified before the Senate Judiciary Committee, alongside former Director of National Intelligence James Clapper, as part of its investigation into Russian interference in last year's presidential election. She said she informed the White House counsel that the Justice Department believed Flynn may have misled officials about conversations with Russian Ambassador to the United States Sergey Kislyak before Trump's inauguration and, as a result, Flynn "essentially could be blackmailed" by the Russian government.

On Feb. 13, Flynn resigned from his position as national security adviser after it was revealed he had misled Vice President Mike Pence and other top administration officials about his conversations with the Russian ambassador.

On March 2, Sessions recused himself from any existing or future probes related to any campaigns for president, after it was reported that the attorney general had two meetings with the Russian ambassador in 2016 after he started supporting Trump's presidential bid and then failed to disclose the contacts during his confirmation hearing.

"There is a pattern here,” Kaine told Stephanopoulos on "GMA" this morning. “There's more to come and we need a special prosecutor.”