In the single biggest political test of his presidency thus far, Trump is looking to carry the Obamacare repeal and replace bill across the finish line.

He made a blunt pitch to House Republicans in a closed-door meeting Tuesday morning on Capitol Hill: You could lose re-election if you don't vote for this legislation.

It was clear, lawmakers said, that Trump was energized by the supporters that had come out for him Monday night in Louisville, Kentucky, as he described the "huge" size of the crowds. That support may not last, he warned.

"We won't have these crowds if we don't get this done," Trump said, according to a source in the room.

He later added: "I honestly think many of you will lose your seats in 2018 if you don't get this done."

According to CNN's ongoing whip count, 19 House Republicans have said they will vote against the bill, while seven more have indicated they are likely to oppose it. GOP leaders can afford to lose 21 members to pass the bill without any Democratic support.

A senior House leadership aide said they are confident they will get the votes, and that House Speaker Paul Ryan is "full involved."

"This is an all-hands-on deck situation. This is a big bill. It's a big promise Republicans made to the American people," the aide said.

The next 48 hours will reveal if Trump's personal lobbying and efforts by Ryan and others will be enough.

Conservative Republicans have emerged among the most fierce critics of leadership's health care bill. Believing that the proposal doesn't go far enough in dismantling the Affordable Care Act, they've dubbed the bill "Obamacare Lite."

Trump has been holding smaller sessions with members of the conservative House Freedom Caucus, whose chairman has opposed the bill.

In Tuesday's all-House GOP meeting, Trump specifically called out Freedom Caucus Chairman Mark Meadows, saying he thinks both Meadows and the group will get on board. "A loss just isn't acceptable," Trump said, according to a source in the room.

The President also said if the Freedom Caucus members don't come along they could face tough primary challenges.

These threats -- described by members as having been delivered in good humor -- were not enough to sway Meadows.

"If this was about the President, I would have changed a long time ago," Meadows said.

Trump's charm

Despite the dire warning about 2018, members leaving the gathering described what they said was vintage Trump -- exuberant, humorous and ready to charm.

"Trump was first rate in every way," said GOP Rep. Tom Cole, a leadership ally. "He was truly spectacular. This is part of the job that he enjoys and excels at -- closing the deal."

GOP Rep. Richard Hudson described Trump's visit as a "tour-de-force."

"He was charming. He was funny. He really did a great job, I think, in letting us know we're in this together," Hudson said. "He's counting on us to vote for this bill."

Trump's political message, Hudson said, was "the American people are watching. They gave us the House, the Senate the White House and if you squander this opportunity I think there will be real consequences for you and the rest of the party."

Texas Republican Rep. Roger Williams, who said he has been "an undecided all along," told CNN after the meeting that he is now "leaning yes" on the GOP health care bill.

"I want this president to succeed and I think America does too, and I'm going to do everything I can to help him," Williams said. "This is big moment in the history of our country to dismantle this horrible Obamacare."

Rep. Darrell Issa, who recently said the legislation was "not in a form I can approve of," said Tuesday that he, too, was leaning yes. Some of his main concerns about the bill were addressed in changes proposed in recent days, the California Republican said.

Others still remain opposed.

In a statement after the Trump meeting, GOP Rep. Ted Budd came out against the legislation.

"This bill leaves the structure of Obamacare in place and does not provide the relief that North Carolina families need from high premiums," Budd said. "I am completely committed to repealing Obamacare and do not think that Congress should take a recess until we have done so."

Late Monday, House leadership announced a set of moderate changes to their health care bill meant to reassure both conservatives and moderates who have a distinct set of concerns about the legislation.

White House press secretary Sean Spicer said more changes could be on the way.

"It's possible," Spicer said. "I think we've made some very positive steps forward, so I don't want to, you know, rule anything out but I will say that I feel very good about this where it stands now."

He added: "The more and more members meet with the President, the more they understand how important this is to the overall agenda."

Nervousness on CBO score

Meanwhile, there is nervous energy among moderate Republicans who fear the political optics of the Congressional Budget Office's assessment that millions more would be uninsured under the GOP bill than under Obamacare.

In advance of Trump's arrival Tuesday morning, Ryan walked members through the changes unveiled Monday, according to a source inside the meeting. His pitch was this was as much as they could do to mollify as many concerns as they could, at this point in the process. He also made clear the reality of their situation: every change in one direction upsets those on the other side of things, so it's a delicate balance, the source said.

This story has been updated with new developments.