Faced with impeachment, Trump's strategy is an all-out offensive meant to turn a grueling public trial into a 2020 campaign tool

• President Donald Trump is almost certain to be impeached on Wednesday, only the third president to undergo the process in US history.
• House Democrats are likely to vote that Trump abused his power by pressuring Ukraine to investigate domestic political rival Joe Biden.
• However, Republicans in the Senate are almost certain to acquit Trump when the issue comes to a trial.
• Faced with this dynamic, Trump and his presidential campaign are seeking to turn the process to their advantage in the 2020 election, using events in Washington to try to sway swing voters ahead of the 2020 election.
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President Donald Trump on Wednesday is almost certain to go down in history as only the third US president to be impeached - the beginning of the formal process to remove a commander-in-chief from office.

Facing the same fate back in 1998, President Bill Clinton projected humility, apologising in a public address in the White House rose garden for his affair with intern Monica Lewinsky.

President Richard Nixon resigned before he could be impeached in 1974, as his full role in the Watergate scandal and subsequent cover-up became clear.

Faced with a stain on his record and the prospect of a grueling public trial, President Donald Trump is taking a totally different route.

In a letter to House speaker Nancy Pelosi on Tuesday night, riddled with factual error and branded by some critics as "unhinged", Trump savaged the impeachment process as a partisan plot to end his presidency.

"More due process was afforded to those accused in the Salem Witch Trials," he wrote, arguing that the House had "cheapened the importance of the very ugly word, impeachment!"

Some critics have likened the letter to one of the president's no-holds-barred speeches at his rallies, rather than a sober riposte during a grave Constitutional process.

And this is likely exactly what Trump intends.

The next step after the House vote would be a trial in the Senate to decide whether Trump must leave office. All indications are that the majority-Republican Senators will acquit the president.

With the final outcome not really in doubt, the question for each side becomes how to use it to their benefit.

For months, Trump and his presidential campaign have been seeking to weaponize the impeachment process, depicting it as a coup by a partisan Congress more interested in scoring political points than advancing the nation.

The campaign has sought to depict impeachment as a desperate, last-ditch attempt by Democrats to remove the president in the wake of the Russia-focused Mueller probe, which the president and his allies claim was a plot by a corrupt establishment of intelligence agents.

Trump has hammered the point home in tweets and at campaign rallies, and will do so again at a rally in Michigan timed to coincidence with a vote in the House of Representatives on impeachment.

His letter put his attacks at the top of news bulletins Tuesday, the eve of the impeachment vote.

One Trump campaign official told Axios that that three years in the White House have made Trump an expert on how to turn bad situations to his advantage.

The campaign has been doing a brisk trade in impeachment related merchandise, selling "Bull-Schiff" t-shirts mocking senior Democrat Adam Schiff and "Where's Hunter?" t-shirts, echoing the president's attacks on Biden's son Hunter Biden.

There is evidence that Trump's message is working in the swing states that he will have to hold if he is to be re-elected.

An average of polls on impeachment since October in battleground states compiled by The Washington Post found that 43% support the process, while 51% oppose.

The president's ability to spin the impeachment proceedings to his advantage has been aided by a conservative media that has relentlessly defended the president's cause - and cocooned him from hugely damaging testimony by key impeachment witnesses.

Trump's letter on Tuesday echoed talking points which Sean Hannity, the Fox News host and one of Trump's most crucial media allies, has for months been pushing on his top-rated show.

It has been promoted, too, in social media ads, where the Trump campaign's spending has outstripped that of any Democrats. This in turn is echoed by a wide network of pro-Trump activists on social media and on the airwaves.

His hope is that the spectacle in Washington will, counter-intuitively, drive support to him in the areas that matter most.

Once upon a time, the scandal of impeachment would likely have spelled doom for a historically unpopular president seeking re-election.

But US political norms that have stood for decades are shattering. Just like in 2016, Trump hopes to turn this new reality to his advantage.