WASHINGTON — The US House of Representatives is working in the middle of the night as Donald Trump and his allies try to pressurise holdouts in the president's own Republican Party into backing his mega-bill on tax and spending.

The sprawling legislation, which could define Trump's second term in office, is nearing its final vote. But it is first undergoing a procedural vote that was called late on Wednesday evening.

Trump's bill has been opposed not only by opposition Democrats, but by a handful of Republicans who criticize its potential on the US deficit, healthcare and other issues.

The bill ground through the US Senate earlier this week in another late-night session. Trump hopes it will pass by Friday.

The House, or lower chamber, approved an earlier version of the bill in May with a margin of just one vote, and this bill, with new amendments that have frustrated some Republicans, must now be reconciled with the Senate version.

Both chambers are controlled by Trump's Republicans, but within the party several factions are fighting over key policies in the lengthy legislation.

The bill narrowly cleared the Senate, or upper chamber of Congress, on Tuesday. Vice-President JD Vance cast a tie-breaking vote after more than 24 hours of debate and resistance from some Republican senators.

It has so far proven equally tricky for Trump's allies to pass the bill through the House.

After about seven hours of wrangling that led to most lawmakers clearing from the chamber on Wednesday, House Speaker Mike Johnson scheduled an evening vote on the rule - a procedural vote that allows the legislation to be brought to the floor for a full vote.

The vote will mark the first test for the legislation and give a barometer on whether Republicans might have the support needed to pass it. A rule vote is typically an easy procedural task, and if passed, will allow debate to start on the legislation.

The president has been very involved in attempting to persuade the holdouts and held several meetings at the White House on Wednesday in hopes of winning them over.

On Wednesday, he took to social media to apply further pressure, saying that the "House is ready to vote tonight". He added that Republicans are "united" to deliver "massive growth".

Ralph Norman, a House Republican from South Carolina, attended one of the meetings but wasn't persuaded.

"There won't be any vote until we can satisfy everybody," he said, adding he believes there are about 25 other Republicans who are currently opposed to it. The chamber can only lose about three Republicans to pass the measure.

"I got problems with this bill," he said. "I got trouble with all of it."

Sticking points include the question of how much the bill will add to the US national deficit, and how deeply it will cut healthcare and other social programmes.

During previous signs of rebellion against Trump at Congress, Republican lawmakers have ultimately fallen in line.

House Majority Leader Steve Scalise told reporters that Republicans were still on their way to Washington to vote, and that several had flight delays due to bad weather.

What is at stake this time is the defining piece of legislation for Trump's second term. But several factions stand in its way.

The Congressional Budget Office (CBO) estimated that the version of the bill that was passed on Tuesday by the Senate could add $3.3tn (£2.4tn) to the US national deficit over the next 10 years. That compares with $2.8tn that could be added by the earlier version that was narrowly passed by the House.

The deficit means the difference between what the US government spends and the revenue it receives.

This outraged the fiscal hawks in the conservative House Freedom Caucus, who have threatened to tank the bill.

Many of them are echoing claims made by Elon Musk, Trump's former adviser and campaign donor, who has repeatedly lashed out at lawmakers for considering a bill that will ultimately add to US national debt.

Shortly after the Senate passed the bill, Texas congressman Chip Roy, of the ultraconservative House Freedom Caucus, was quick to signal his frustration.

He said the odds of meeting Trump's 4 July deadline had lengthened.

Freedom Caucus chairman Andy Harris of Tennessee told Fox News that Musk was right to say the US cannot sustain these deficits. "He understands finances, he understands debts and deficits, and we have to make further progress."

On Tuesday, Conservative congressman Andy Ogles went as far as to file an amendment that would completely replace the Senate version of the bill, which he called a "dud", with the original House-approved one.

Ohio Republican Warren Davison posted on X: "Promising someone else will cut spending in the future does not cut spending." — BBC