U.S. Democratic House Speaker Nancy Pelosi denounced the Senate's acquittal of President Donald Trump on Wednesday and said he remains "an ongoing threat to American democracy".

"Today, the President and Senate Republicans have normalized lawlessness and rejected the system of checks and balances of our Constitution," Pelosi said in a statement issued after the Senate acquitted Trump of both impeachment articles passed by the House.  

"The President remains an ongoing threat to American democracy, with his insistence that he is above the law and that he can corrupt the elections if he wants to," Pelosi said.  

Senate Democratic minority leader Chuck Schumer said Trump's acquittal by the Senate was "virtually valueless" since Republicans refused witnesses at his impeachment trial.    

"Now that Republicans have rejected a fair trial, truth is a giant asterisk next to the president's acquittal," Schumer told reporters.  

"The asterisk says he was acquitted without facts. He was acquitted without a fair trial. And it means his acquittal is virtually valueless," the senator from New York said. 

"The White House may be cheering this as a win," Schumer said, but "history will view this as a Pyrrhic victory for Senate Republicans, for the Republican Party and for President Trump." 

The Republican-majority Senate voted 52-48 to acquit Trump of abuse of power and 53-47 to acquit him of obstruction of Congress.

The White House welcomed the Senate vote to acquit the Republican president, calling the impeachment process a "witch hunt... based on a series of lies."

"Today, the sham impeachment attempt concocted by Democrats ended in the full vindication and exoneration of President Donald J. Trump. As we have said all along, he is not guilty," the White House press secretary said in a statement. 

Senate majority leader Mitch McConnell said the Democratic Party's failed effort to impeach and remove President Trump was a major political miscalculation likely to benefit Republicans. 

"Right now this is a political loser for them. They initiated it. They thought this was a great idea," McConnell told reporters. "At least for the short term, it has been a colossal political mistake."

In December, the Democrat-led House of Representatives voted to impeach Trump, making him the third president in U.S. history to be formally impeached and face a trial in the Senate.

Trump emerged confident and ready to press on with his re-election effort on Wednesday after the Democratic-led impeachment drive that he denounced as illegitimate crashed to a halt in the Republican-led Senate. 

Trump plans to speak about the issue at the White House on Thursday. A source close to the president described his address as a "vindication speech" that would combine some magnanimity with an "I told you so" tone. 

Next, advisers said, Trump would proceed at full steam on his political and policy goals, throwing himself fully into his re-election campaign and efforts to fulfill promises he has made to his supporters and the electorate. 

"The president is pleased to put this latest chapter of shameful behavior by the Democrats in the past, and looks forward to continuing his work on behalf of the American people in 2020 and beyond," the White House said in the statement. 

Trump was acquitted largely along party lines on two articles of impeachment that accused him of abusing his power by pressing Ukraine to investigate Joe Biden, a contender for the Democratic nomination to face him in the November 3 election, and obstructing Congress' attempts to investigate the matter. 

But he did not come out of the process unscathed. As Pelosi made clear, the impeachment will be part of his legacy, and Republican Senator Mitt Romney's vote to convict him on the abuse-of-power charge deprived the president of the ability to dismiss the process as entirely partisan.

But Republican officials noted record fundraising during the impeachment process, leading Trump's re-election effort to bring in 155 million U.S. dollars in the last three months of 2019 alone, boosted by a support base that is both pumped up and ticked off. 

Although the bruising impeachment battle is certain to be a factor for voters considering whether to re-elect Trump in November, his campaign is claiming victory. 

"Acquittal means total vindication," said Tim Murtaugh, the Trump campaign's communications director. "The Democrats' decision to move forward with impeachment will go down as the worst political miscalculation in American history." 

In a sign of confidence, minutes after senators found him not guilty, the president tweeted a video of himself with campaign signs projected well into the future, suggesting he wanted to be president for decades to come. 

U.S. presidents are constitutionally limited to two elected terms in office. 

Trump released another video several hours later that referred to Romney as a "Democrat secret asset" and said the senator tried to "infiltrate" the president's administration when Trump considered him for the position of secretary of state. 

(With input from AFP, Reuters)