Trump's trouble with Senate moderates, and what that means for health care

— -- The process of wrangling 50 Republican votes both for the Senate's proposed replacement health care plan as well as a straight repeal of Obamacare has so far proved impossible for Majority Leader Mitch McConnell, R-Ky., and President Donald Trump.

Part of the trouble on healthcare stems from the factions within the Republican caucus of 52 senators, which provides the GOP's razor-thin margin in the chamber. The balancing act comes from the ideological splits within the Republican caucus in the Senate, as at least 10 of the 52 current Republicans are occasionally viewed as moderates on certain issues, while the remaining lawmakers are conservative of varying stripes.

An earlier version of the Senate bill was stymied after at least nine senators came out against it. The bill was adjusted to accommodate concerns, but at least four Republican senators said they would not vote in favor of it. Then after that, three moderate senators said that they wouldn't vote in favor of the procedural motion to bring a repeal vote to the floor and set the deadline for a replacement plan for two years.

John Hudak, a political scientist and senior fellow at the Brookings Institution, told ABC News that further tweaks to the replacement plan are not likely to help pull together a group of 50 of the 52 Republicans on the issue.

"If you move more to the middle, you lose people on the right," Hudak said.

"There's seemingly no combination where you get 50 votes in the middle of the conference where you only lose one person on either side of the party ... you can't move it where you only lose two people on one end," he said.

The 50-vote threshold factors in a tie-breaking vote from the vice president and comes under Senate rules that allow the majority to bypass the filibuster.

The only option Hudak sees as a mathematical possibility would be to focus on Democrats, but it "would be an all-out assault on Senate Republicans by Republicans."

"There probably are 50 votes in the Senate but you'd need 40 Democrats. You'd probably find five to 10 Republicans that could go for a fix of Obamacare ... but the political costs for McConnell are too high for him to enjoy the policy benefits," Hudak said.

And while Trump has been having both small and larger groups of senators over to the White House for coalition-building meals, Hudak doesn't think that will help.

"Mitch McConnell is 100 times more talented than the president when it comes to legislative coalition-building, so if Mitch McConnell can't do, Donald Trump will never be able to do it," Hudak said.

If there is a group that Trump may have more sway with, it's conservatives, says Hans Noel, an associate professor at Georgetown University.

"Even though he is not himself very conservative, most of his voters identify as conservative. Conservative elites and commentators don’t like him, but voters do," Noel said.

"They are also the ones who are most concerned about crossing Trump, because it is their voters who supported Trump and who might support a primary challenge. So conservatives are more likely to go along with him. Moderates are more likely to be from districts where Trump is not as popular, and are less likely to be worried about crossing his voters," he said.

But all of the Senate conservatives are currently on board with - or at least have not come out against - the latest version of the Republican Senate health care plan, meaning the only direction McConnell and Trump can turn to try to secure 50 votes is to the center of the party.

And for those moderates who oppose the current version of their plan, the possibilities of primary challenges is not unfounded: the Senate Conservatives Fund, a grassroots group that has a history of supporting primary challengers, sent out a release on Tuesday saying that they are seeking challengers for any "Obamacare Republicans" who vote against a repeal.

Trump himself didn't shy away from the suggestion of encouraging challengers for people who don't get in line, joking with Sen. Dean Heller, R-Nev., who was seated next to him at the lunch. Heller opposed the original Senate health care bill, siding with his centrist Republican governor Brian Sandoval over concerns for Nevadans who benefited from the state's acceptance of the Obamacare Medicaid expansion. He has not indicated how he would vote to advance the new GOP plan.

"This was the one we were worried about, you weren't there," he said, motioning to Heller. "You're going to be. Look, he wants to remain a senator, doesn't he?"

Trump added: "Any senator who votes against starting debate is really telling America that you're fine with Obamacare."

Supporters of his efforts during the health care reform process, which include his former rival Sen. Ted Cruz, R-Texas, dismiss the comments.

"Listen, the president has his own way of communicating," Cruz said after the lunch meeting.

Hudak thinks such veiled threats aren't helpful.

"The public side of what the president has been doing has just been a total disaster. His tweeting or his suggestions that he'll [support] primary [challengers against] members of his own party ... he's not making any friends," Hudak said.

Burning those bridges, or at least charring them, may be a business tactic that is not helpful for Trump to carry over from his business empire and transfer into Washington, Noel said.

"I think a lot of the problem is simply that Trump isn’t very experienced with the kind of negotiations that are needed to pass a bill. Even if you believe he’s good at making deals in real estate, legislation is something entirely different. You can’t walk away from a bad deal and go looking somewhere else. You always have to deal with the same legislators," Noel told ABC News.