New York: President Donald Trump has been acquitted by the Senate on both impeachment charges ending the historic trial that was a desperate move by Democrats to avenge their defeat in the 2016 election.

While the Democrats put up a united front to vote to convict him, one Republican, Mitt Romney, defected to the Democrats to vote guilty on Wednesday on one of the charges.

Supreme Court Chief Justice John Roberts, who presided over the trial, solemnly announced Trump's acquittal after the vote was 52 votes to acquit and 48 to convict on the first charge of abuse of power and 53 not guilty votes to 47 guilty on the second charge of obstruction of Congress.

Vice President Mike Pence said that after months of "sham investigations" and partisanship Trump has been acquitted.

Trump was the third president impeached in the nation's 243-year history, and like him Andrew Johnson was acquitted in 1868 and Bill Clinton in 1998, with the Senate unable to come up with the two-thirds majority required to convict.

But in the deeply polarised political universe, the Democrats were not ready to give up.

Jerry Nadler, the chair of the House Judiciary Committee, who framed the charges against Trump vowed to continue the pursuit.

He said that the House would consider summoning fired National Security Adviser to testify even though the Senate turned down the Democrat prosecutors' request to call him as a witness.

The Democrat-controlled House of Representatives voted in December on the two charges after three months of secret and public hearings and debates and sent the article of impeachment containing the accusations to the Senate for the trial after holding back for a month in an unsuccessful bid to negotiate the trial procedures.

As was clear from the day one after his election, when Democrats first made the threat of impeachment, there was no chance of him being convicted with the required two-thirds majority by the Senate.

Therefore, the vote to acquit Trump was not a setback to the Democrats, who pursued the impeachment as a catharsis for Hillary Clinton's defeat in 2016 and a campaign ploy to discredit him in the November election.

Trump was charged with abuse of power over a request he made to Ukraine President Volodymyr Zelentsky to investigate that activities of former Vice President Joe Biden and his son, Hunter, in that country.

He was also accused of withholding nearly $400 million in military aid to Ukraine to pressure Zelentsky to conduct the probe.

Because he refused to allow his officials to testify and provide documents the House investigators requested, the obstruction of Congress charge was added.

Hunter Biden, who left the Navy amid drug abuse allegations, was made a director of a Ukrainian gas company with monthly payments of more than $50,000.

His father, who was overseeing Washington's relations with Kyiv, later said that he had the prosecutor looking into that company dismissed.

Democrats said that asking for the probe amounted to seeking foreign interference in US elections as Biden was the front-runner for the Democratic Party nomination run against Trump in November.

Ironically, while the trial was on in the Senate, Biden's popularity slipped and he ended fourth in Iowa on Monday in the first Democratic Party election process for the presidential nomination, and is lagging in the opinion polls in the New Hampshire election next week.