President Donald Trump rang in the anniversary of the appointment of special counsel Robert Mueller by slamming the probe into his campaign's ties to Russia during the 2016 election.

"Congratulations America, we are now into the second year of the greatest Witch Hunt in American History ... and there is still No Collusion and No Obstruction," Trump tweeted. "The only Collusion was that done by Democrats who were unable to win an Election despite the spending of far more money!"

Congratulations America, we are now into the second year of the greatest Witch Hunt in American History...and there is still No Collusion and No Obstruction. The only Collusion was that done by Democrats who were unable to win an Election despite the spending of far more money! — Donald J. Trump (@realDonaldTrump) May 17, 2018

Later Thursday, his re-election campaign sent out a fundraising email, citing the one-year anniversary of Mueller's appointment as a reason for supporters to donate.

"On this anniversary of the greatest Witch Hunt in American history, I am asking you to renew your Sustaining Membership for the 2018 year to show the witch hunters that *NOT A SINGLE PATRIOT* backed down from our fight," the email said.

Thursday marked one year since Deputy U.S. Attorney General Rod Rosenstein appointed Mueller, a former FBI director, to be a special counsel overseeing the investigation.

Despite the disgusting, illegal and unwarranted Witch Hunt, we have had the most successful first 17 month Administration in U.S. history - by far! Sorry to the Fake News Media and “Haters,” but that’s the way it is! — Donald J. Trump (@realDonaldTrump) May 17, 2018

Following Mueller's appointment, Trump said he looked forward "to this matter concluding quickly."

"As I have stated many times, a thorough investigation will confirm what we already know — there was no collusion between my campaign and any foreign entity," he said in the May 17, 2017, statement. "I look forward to this matter concluding quickly. In the meantime, I will never stop fighting for the people and the issues that matter most to the future of our country."

The probe has not gone as he'd predicted.

Since Mueller's appointment, nearly 20 individuals have been charged with crimes, including five who pleaded guilty (among them Michael Flynn, the former national security adviser) and 13 Russian nationals.

Trump has repeatedly ripped the probe as a "witch hunt" and has reshuffled his legal team multiple times. Earlier this month, Trump tapped Emmet Flood, who advised Bill Clinton during his impeachment proceedings, to assist in the Russia investigation. And last month, he brought on former New York Mayor Rudolph Giuliani, a longtime Trump ally and ex-federal prosecutor in New York, to provide advice on how to deal with the special counsel's probe.

How — or when — the probe will end, however, remains unclear.

Giuliani said Wednesday that Mueller's office had told Trump's legal team that he won't indict a sitting president.

"They (the special counsel's office) acknowledge the fact that they can't indict us," Giuliani told NBC News, indicating that the information had been conveyed to Trump's lawyers. "They know they don't have that power. So their function is to write a report. We would like it to be the fairest report possible. But even if it isn't, we're prepared to rebut it in great detail, so we'd like them to do it."

He added: "It's as clear as can be that they don't have the right to indict under the Justice Department rules. And I know they're not going to indict." Giuliani also suggested Wednesday that Mueller may not even interview Trump.

Senate Minority Leader Chuck Schumer, D-N.Y., criticized Trump Thursday for constantly characterizing Mueller's investigation as a "witch hunt."

"It's not a witch hunt when some of the most senior members of the Trump campaign have been indicted," Schumer said on the Senate floor. "It’s not a witch hunt when Democrats and Republicans agree with the intelligence community that Russia interfered in our election to aid President Trump."

On Wednesday, the Senate Intelligence Committee concluded that Russia had interfered in the 2016 presidential election with the intention of helping Trump.

"Any fair minded citizen, even the most ardent partisan, should be able to look at the facts and say that this investigation is not a witch hunt," Schumer added. "We should all be aghast on this one-year anniversary of Mueller’s appointment at the smear campaign by the president and his allies."

House Speaker Paul Ryan, R-Wis., for his part, said Thursday that Mueller should be "free to do his job" but noted that he would also like to see the probe come to an end.

"We want to see this thing come to its conclusion but again I have always said, he should be free to finish his job," Ryan said.