Washington (CNN) President Donald Trump warned House Republicans Tuesday if they can't pass health care legislation after seven years of promises it could be a "bloodbath" in the 2018 midterm election, according to one member present in the meeting.

Trump vowed to throw his full support behind the effort to repeal and replace the Affordable Care Act during a meeting with House GOP leadership, saying he is "proud" to support a GOP-authored plan to replace Obamacare and told members behind closed doors that he would support it "100%," according to sources in the room.

But he warned lawmakers of the high-stakes nature of the effort, citing a potential electoral "bloodbath," a member present said.

"He said he hopes members understand that," the source said.

Despite early opposition from conservative groups and some members of Congress, there was a lot of optimism among the House leadership members charged with passing the legislation that members would come around.

Trump promised House Republicans in the meeting he would use the power of the bully pulpit -- calling and meeting with members -- to get conservatives and moderates in line and pass the bill.

"I'm proud to support the replacement plan released by the House of Representatives and encouraged by members of both parties," Trump said during brief remarks captured by the press pool.

Republicans first unveiled their legislation, which would eliminate the individual mandate to obtain health insurance contained in Obamacare and include tax incentives meant to encourage Americans to purchase coverage, Monday night.

The gathered lawmakers told the President they faced obstacles ahead without any expectation of Democratic support for their plan, nor any idea of where some conservatives might land. The House GOP vote-counters told Trump they planned to move forward with a whip count later this week

"We don't have it today, but I think we'll get it," a source in the room told CNN.

Trump's remarks during the roughly 30-minute meeting with members in the East Room of the White House reflected the most explicit administration endorsement yet of the replacement health care plan.

"I cannot comment on the discussions in closed door meetings," said Lindsay Walters, a White House spokeswoman, when asked about Trump's comments about the political pitfalls of failing to pass a replacement plan.

Trump had not yet weighed in overtly on the Republican plan until Tuesday afternoon. Earlier, he tweeted support for the bill but also suggested there were elements of the measure that could be changed.

JUST WATCHED Spicer compares healthcare bills by size Replay More Videos ... MUST WATCH Spicer compares healthcare bills by size 01:23

"Our wonderful new Healthcare Bill is now out for review and negotiation," he wrote Tuesday morning.

Sources in the room told CNN that Trump did not explicitly call out any member or group opposed to the bill, but did emphasize the importance of getting the job done and working together to do it.

"We cannot toss this out and start all over. We are too far down the road for that," the source said.

The whip strategy going forward appears to be to remind members again and again that voting against the House's bill is the same as voting against their President, and handing a victory to Senate Minority Leader Chuck Schumer and House Democratic Leader Nancy Pelosi. The strategy is to remind members there is no second chance to get this right.

Vice President Mike Pence also joined the meeting and was feeling hopeful about the legislation's prospects in the Senate, sources said, despite the fact that two senators have already come out with deep concerns about the House legislation.

While Trump said he was committed to the bill, he did indicate he was open to making changes. Sources in the room, however, said that the changes would have to win broad support and be passed through the amendment process, making it difficult to amend the contours of the legislation in any meaningful way.

"This is a guy who knows how to close a deal, and he thinks we are at the point were we need to close," the source said.

Trump took to Twitter Tuesday evening to reach out to one of the bill's most vocal conservative opponents.

"I feel sure that my friend @RandPaul will come along with the new and great health care program because he knows Obamacare is a disaster!" Trump tweeted, just as an interview with Sen. Rand Paul was about to air on CNN's Erin Burnett "OutFront."

I feel sure that my friend @RandPaul will come along with the new and great health care program because he knows Obamacare is a disaster! — Donald J. Trump (@realDonaldTrump) March 8, 2017

In that interview taped prior to Trump's tweet, the Kentucky Senator said he met with Trump and didn't believe the two were "that far off" from each other on how to proceed. Paul said they both want to repeal Obamacare, and the disagreement came over how exactly to replace it -- and whether or not that replacement should be combined with the repeal in one bill.

"I did express with the President that I think separating repeal from replacement will get it done, and I think that's maybe the only vehicle for getting that done," Paul said.