WASHINGTON (AFP) - President Donald Trump goes to his State of the Union speech on Tuesday (Feb 4) gloating over the Democrats' chaotic Iowa caucuses, sure of acquittal in his impeachment trial, and more optimistic than ever of re-election.

The Republican is turning what could have been the darkest week of his presidency - with years of scandals crescendoing into only the third presidential impeachment trial in US history - into a victory lap.

And the Iowa Democrats' mess up of the start of the primary season gifted him yet another boost ahead of November's presidential election.

"Nothing works, just like they ran the country," Trump said on Twitter.

The State of the Union is a grand set-piece in the US political calendar, a rare occasion when bitter opponents traditionally observe a truce while the president lays out a vision for the future.

Trump, however, will motorcade up Pennsylvania Avenue to the Capitol on the eve of his impeachment verdict and at a time of national rancor arguably unmatched for decades.

The White House promises the president will deliver an upbeat message on taking the podium at around 9pm (10am on Wednesday, Singapore time).

"I think the speech is going to have a very optimistic tone," a senior aide, who refused to be identified, told reporters.

But judging from the White House's preview, the address will sound more like a campaign speech, with praise for Trump's own policies, than a bid to bring the country together.

The main focus will be the strong US economy and the "blue collar boom," with the overall theme being dubbed the "great American comeback" - a phrase echoing Trump's campaign slogan, "Make American Great Again."

UNREPENTANT TRUMP

The venue will be the House of Representatives, where just last December the Democratic majority impeached the president for abuse of power and obstructing Congress.

Sitting a few feet away will be House speaker Nancy Pelosi, whom Trump repeatedly insults in public, calling her "Nervous Nancy" and other nicknames.

A sea of hostile Democratic legislators will watch him from the seats, as will the Republicans who hold a majority in the Senate and are all but sure to declare him not guilty in the impeachment verdict vote on Wednesday.

The speech could in theory be an opportunity to heal a nation boiling over in mistrust.

Trump could express regret for what even several of his own Republican senators publicly say was wrongful behavior in pushing Ukraine to open a questionable corruption probe against one of his main Democratic presidential opponents, Joe Biden.

Or he could seek to calm the waters by entirely avoiding the topic of impeachment, just as Bill Clinton did during his post-impeachment State of the Union in 1999.

White House officials say they don't know what he'll do.

"It's never safe to assume anything," the aide said, when asked if impeachment would be kept out of the speech.

Republican Senator Roy Blunt told The New York Times Trump should "avoid" the "I" word.

"It's an opportunity to move on," Blunt said. "The other option is to address it head on - and he is often a head-on kind of guy."

NO END IN SIGHT

Even though the impeachment proceedings will end with Wednesday's verdict vote, the Democrats say they'll keep going at Trump, whom they accuse of seeking to cheat in the upcoming election.

"The plot goes on, the scheming persists and the danger will never recede," Adam Schiff, the lead impeachment manager, said Monday, challenging Republicans to "speak the awful truth."

While the Senate is safe ground for Trump, the Democratic majority in the House is expected to continue aggressive probes of Trump's alleged corruption.

Meanwhile, the main fight is expected to shift to the campaign trail ahead of November.

Trump has barely stopped campaigning since the day he took office in 2017 and his team boasts strong funding and organisation.

The Democrats, by contrast, are deeply divided between the left wing of Bernie Sanders and moderates like Biden.

Their cause suffered another setback with the organisational confusion in Iowa, an event that was meant to have showcased the party's energy and hunger to take on Trump.

Next week, it will be New Hampshire holding a primary and from there the pace of the competition to get the party nomination will pick up.

Trump predicts he will trounce his eventual opponent. He got yet more good news on Tuesday with a Gallup poll showing his approval rating at its highest ever: 49 per cent.

Support was up both among Republicans and independents, though opposition among Democrats had deepened more than ever, the poll found.