Senate Democrats slammed President Trump on Tuesday for firing FBI Director James Comey amid the bureau's investigation into ties between the Trump campaign and Russia, drawing comparisons to former President Richard Nixon.

"This is Nixonian,” Sen. Bob Casey Robert (Bob) Patrick CaseyGAO report finds brokers offered false info on coverage for pre-existing conditions Catholic group launches .7M campaign against Biden targeting swing-state voters GOP senator to quarantine after coronavirus exposure MORE (D-Pa.) said in a statement.

Sen. Patrick Leahy Patrick Joseph LeahyBattle over timing complicates Democratic shutdown strategy Hillicon Valley: Russia 'amplifying' concerns around mail-in voting to undermine election | Facebook and Twitter take steps to limit Trump remarks on voting | Facebook to block political ads ahead of election Top Democrats press Trump to sanction Russian individuals over 2020 election interference efforts MORE (D-Vt.) echoed that language, noting that Trump fired Comey "in the midst of one of the most critical national security investigations in the history of our country — one that implicates senior officials in the Trump campaign and administration."

"This is nothing less than Nixonian," Leahy said.

Trump said he fired Comey at the recommendation of Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsTrump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status White House officials voted by show of hands on 2018 family separations: report MORE and Deputy Attorney General Rod Rosenstein.

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“While I greatly appreciate you informing me, on three separate occasions, that I am not under investigation, I nevertheless concur with the judgment of the Department of Justice that you are not able to effectively lead the bureau,” Trump wrote in a letter to Comey dated Tuesday.

Casey compared Comey’s firing to Nixon and top officials in his administration going to great lengths to cover up the Watergate break-in as well as other illegal activities authorized by the Nixon White House.

Nixon resigned when he faced the possibility of being impeached by the House of Representatives following the Watergate scandal.

Casey called on Rosenstein to “immediately appoint a special counsel to continue the Trump/Russia investigation.”

Democrats raised questions about whether the firing was aimed at trying to sway the FBI's Russia investigation, with many questioning the timing of the dismissal.

Casey wasn't the only person to draw a comparison between Nixon and Trump, with other lawmakers and political observers saying the decision to fire Comey was a call back to the Watergate investigations:

I'm no fan of FBI Comey, but firing him in midst of Russia probe raises very serious concerns. Remember result of Nixon-Cox. — Rep. Michael Capuano (@RepMikeCapuano) May 9, 2017