Donald Trump said he hopes for a "truly honest" outcome from the investigation into Russian meddling in the presidential election after news broke that a grand jury has been convened.
The president challenged Democrats to either continue their "obsession with a hoax" or begin serving the interests of the American people.
At a boisterous campaign rally in West Virginia, the president slammed the investigation as a "fake story that is demeaning to all of us and most of all demeaning to our country and demeaning to our constitution".
He commented hours after news broke that Robert Mueller, the special counsel leading the investigation, had empanelled a grand jury in the case.
"I just hope the final determination is a truly honest one, which is what the millions of people who gave us our big win in November deserve and what all Americans who want a better future want and deserve," Mr Trump told supporters in Huntington.
Mr Trump overwhelmingly won the state in the November election, partly due to his promises to revive a slumping coal industry.
The president's lawyers had earlier said they were unaware of the existence of a grand jury and had no information to suggest Mr Trump himself was under investigation.
The president, who remains agitated over the investigation into allegations of co-ordination between his campaign associates and Russian government officials, told the rally Democrats have a decision to make.
"They can continue their obsession with the hoax or they can serve the interests of the American people," he said.
Mr Trump maintains there were no ties between his campaign and Russia and says no wrongdoing was committed.
His frustration over the investigation peaked in recent weeks as he began attacking Attorney General Jeff Sessions for recusing himself from the investigation. Mr Sessions, most recently a US senator, was an early and ardent Trump supporter.
"Most people know there were no Russians in our campaign. There never were," Mr Trump told the cheering crowd in West Virginia.
"We didn't win because of Russia. We won because of you."
Mr Trump argued that Democrats are pushing the "totally made-up Russia story" because "they have no message, no agenda and no vision".
He said: "The Russia story is a total fabrication. It's just an excuse for the greatest loss in the history of American politics," he said, referring to his victory over Hillary Clinton.
"It just makes them feel better when they have nothing else to talk about."
And he told Democrats: "Try winning at the voter booth. Not going to be easy, but that's the way you're supposed to do it."
At the rally, West Virginia Governor Jim Justice announced that he is deserting the Democrats and joining the Republicans.
"This man is a good man. He's got a backbone. He's got real ideas," Mr Justice said about the president.
"He cares about America. He cares about us in West Virginia."
The president has been holding campaign-style events on friendly turf to get out of Washington and recharge in front of boisterous supporters.
West Virginia voted overwhelmingly for him last year, and the event in Huntington follows recent policy announcements liked by Mr Trump's conservative base.
He recently announced a ban on transgender people serving in the US military, and he also endorsed legislation that aims to slash legal immigration to the US.
The president held a campaign rally last week in Ohio.