The Republican-led Unites States Senate is expected to acquit President Donald Trump on Wednesday at the end of his impeachment trial on charges that he abused his power in dealings with Ukraine and obstructed efforts to uncover the alleged misconduct.

Here is a summary of the reasons that Trump's Republicans, who control 53 seats in the 100-seat chamber, say he should not be removed from office:

Trump did nothing wrong

The impeachment charges against Trump contend that he sought to pressure Ukraine to investigate former Vice President Joe Biden, a leading 2020 Democratic presidential contender, to benefit his own re-election campaign.

Trump impeachment: What you need to know ahead of the vote

They say Trump withheld nearly $400 million in US security aid and a coveted White House meeting with the newly elected Ukrainian president, Volodymyr Zelenskiy.

Some Republicans say the US president did nothing wrong. They say Trump was simply trying to crack down on corruption in a country where that has long been a problem and wanted US allies to share the burden of supporting Ukraine.

"Both of those objectives are consistent with law, are permissible and legal," Republican Senator Ted Cruz said on the Senate floor.

Senators making this argument tend to represent reliably conservative states or, like Cruz, do not face re-election this year.

Trump's actions were wrong, but not impeachable

Nearly half-a-dozen Senate Republicans, including some from electoral swing states such as Pennsylvania and Ohio, say Trump's actions were wrong but do not qualify as "high crimes and misdemeanours", which the US constitution specifies as grounds for impeachment.

"The president did it, shouldn't have done it. But it's a far cry from what the constitution sets out as the standard for removing a president from office," said Republican Senator Lamar Alexander of Tennessee.

Others making that argument include Susan Collins, a moderate Republican facing a tough re-election campaign in Maine. She told the Senate that Trump's request for an investigation of Biden was "improper and demonstrated very poor judgment".

Removal of Trump would upset voters

Regardless of the merits of the impeachment case, a large number of Republicans say ousting the president from office could worsen partisan divisions.

"Can anyone doubt that at least half of the country would view his removal as illegitimate as nothing short of a coup d'etat?" Republican Senator Marco Rubio of Florida said in a statement.

No Republicans voted for Trump's impeachment in the Democratic-led House of Representatives in December. During the Senate impeachment trial, Trump's lawyers accused Democrats of seeking to remove Trump from office even before he became president in January 2017.

Trump's approval rating has shown little change since news broke of his efforts to pressure Zelenskiy in September, and he remains popular among Republican voters. Reuters/Ipsos polling shows that Trump's approval rating stood at 39 per cent at the end of last week, down from 43pc in late September, which is not a statistically significant change.

Trump's popularity among Republican voters is surely a factor for the 21 Republican senators seeking re-election this year, as they could face a backlash if they were to vote to convict.

Not enough evidence

Republicans accuse House Democrats of bringing a "half-baked" impeachment case to the Senate, saying they failed to fight in federal court for vital witnesses and documents that Trump has withheld.

They say House investigators have since inappropriately tried to persuade the Senate to complete the task for them by subpoenaing additional witnesses and documents. All but two Republicans voted last week against a Democratic motion to call more witnesses and present more evidence that could help make the case.

"They claimed dozens of times, that their existing case was, quote, 'overwhelming and incontrovertible'," Senate Majority Leader Mitch McConnell said. "At the same time, they were arguing for more witnesses."

Other Republicans say the impeachment case relies too heavily on unprovable assertions that Trump's motives were corrupt. They say it could set a precedent that would allow a future Congress to punish a president for pursuing genuine anti-corruption policies.

"The House of Representatives' abuse-of-power theory rests entirely on the president's subjective motive. This very vague standard cannot be sustained," said Republican Senator Chuck Grassley.

Let the voters decide

Republicans frequently said impeachment would subvert the will of voters who elected Trump in 2016.

They say the Senate should not interfere with the November 3 presidential election, in which Trump will seek another four years in office.

"Under the constitution, impeachment wasn't designed to be a litmus test on every action of the president. Elections were designed to be that check," Republican Senator Joni Ernst said.