Republican leaders in the US Congress were on the brink of another humiliating health care defeat Tuesday, as they struggled to wrangle enough votes to pass Donald Trump's latest bid to repeal and replace Obamacare.

An earlier version of the Republican plan collapsed in March, when opposition from both moderates and conservatives torpedoed their own party's attempt to do away with Barack Obama's Affordable Care Act.

The president and the White House have touted an amended version of a multiple times in recent days, stressing that despite divisions within the Republican Party, they were gaining support toward reaching the 216 votes necessary to pass the House of Representatives.

"We're making very good progress," insisted House Speaker Paul Ryan.

But with all Democrats expected to vote no on the plan, and a prominent Republican announcing his opposition Tuesday, those advances were in jeopardy.

Republicans can afford only 22 defectors in order for the bill to pass, and one whip count, by NBC News, put the Republican "no" votes at 20, with a dozen more undecided.

A 21st, congressman Fred Upton, came out opposed, saying he was uncomfortable with a new amendment which allows states to remove essential health benefits, including protections that require insurance providers to offer coverage regardless of pre-existing conditions.

The amendment was crucial in bringing reluctant conservatives on board. But some Republicans remained unwilling to back a bill that waters down the pre-existing conditions provision, a popular element of Obama's signature .

"There are a good number of us that have raised real red flags and concerns, and it's not going to get my yes the way that it is," Upton told WHTC radio.

On Monday, perhaps sensing the amended bill was in trouble, Vice President Mike Pence trooped up to Congress and huddled with Republicans.

Confusion surrounded the new version of the legislation, especially after Trump himself insisted Sunday that the new bill would "beautifully" protect people with pre-existing conditions.

He also said that "high-risk pools" would stabilize costs for the sickest individuals.

The American Medical Association has urged Congress to oppose the new plan, warning that the bill provides "no certainty" that the risk pool system would prevent discrimination against people with high-cost medical conditions.