US President Donald Trump on Saturday claimed his polling numbers were up after the Senate paved the way for his acquittal next week on impeachment charges of abuse of power.

The Senate on Friday rebuffed Democratic calls for new witnesses at Trump's trial, with a vote on whether to acquit him due on Wednesday -- the day after he gives his annual "State of the Union" speech.

Trump is all-but-assured of being acquitted by the Senate, where Republicans hold 53 seats to 47 for the Democrats. A two-thirds majority, or 67 senators, is needed to remove a president from office.

"Trump poll numbers are the highest since election, despite constant phony Witch Hunts!" the president tweeted from his Mar-a-Lago resort in Florida. "Tens of thousands of people attending rallies... Fun because USA is WINNING AGAIN!" he added.

Trump currently has a 44.6 percent approval rating in collated polls, almost exactly the same as when he came to office in January 2017, according to the RealClearPolitics website. Democratic Party contenders competing to take on Trump in November's presidential elections campaigned on Saturday in Iowa ahead of the state's all-important caucuses on Monday. The outcome could provide a major sign whether Joe Biden has maintained his front-runner status over Bernie Sanders, Elizabeth Warren and Pete Buttigieg.

On Friday, just two Republican senators -- Mitt Romney of Utah and Susan Collins of Maine -- joined Democrats in voting to introduce further witnesses into the impeachment trial. Democrats thus failed to muster the four Republican votes needed to allow testimony from Trump's former national security advisor John Bolton, acting White House chief of staff Mick Mulvaney and others. Senate Democratic leader Chuck Schumer called it a "grand tragedy."

"America will remember this day -- a day when the United States Senate did not live up to its responsibilities, turned away from truth and instead went along with a sham trial," Schumer said. Nancy Pelosi, the Democratic speaker of the lower chamber House of Representatives, which impeached Trump on December 18, accused Republicans of being "accomplices to the president's cover-up."

"He is impeached forever," Pelosi said. "There can be no acquittal without a trial. And there is no trial without witnesses, documents and evidence."

Democrats had been eager to hear from Bolton following reports that he claims in an upcoming book to have been personally told by Trump that military aid to Ukraine was tied to Kiev investigating former vice president Biden. The charge is the crux of the case that led to Trump becoming just the third president in US history to be impeached by the House of Representatives.

Republican Senate Majority Leader Mitch McConnell, a Trump ally, said House prosecutors, known as "managers," had already presented enough evidence to make their case and there was no need for further witnesses. "A majority of the US Senate has determined that the numerous witnesses and 28,000-plus pages of documents already in evidence are sufficient to judge the House managers' accusations and end this impeachment trial," he said.

McConnell said the Senate, the upper chamber, would resume as a court of impeachment on Monday to hear final arguments, before voting on Wednesday on the two articles of impeachment passed last month by Democrats in the House.

Democrats had hoped to sway enough Republicans to support the call for further witnesses but efforts collapsed when Alaska Senator Lisa Murkowski said she would vote "no" to new testimony. "Given the partisan nature of this impeachment from the very beginning and throughout, I have come to the conclusion that there will be no fair trial in the Senate," Murkowski said.

Republican Senator Lamar Alexander of Tennessee, who had also been considered a possible swing vote, also said there was no need for more evidence. "It was inappropriate for the president to ask a foreign leader to investigate his political opponent and to withhold United States aid to encourage that investigation," Alexander said.

"But the Constitution does not give the Senate the power to remove the president from office and ban him from this year's ballot simply for actions that are inappropriate," he said. Four contenders for the Democratic nomination -- Sanders, Warren, Amy Klobuchar and Michael Bennet -- were stuck in Washington in the run-up to the Iowa caucuses as they sit in the Senate and were unable to leave due to the impeachment trial.