House Speaker Paul Ryan could point to any number of challenges as his biggest to overhauling Obamacare. His answer: Senate procedure.

"People are saying I’d love to have this in there, I’d love to have that in there," Ryan says.

But he can't. Ryan knows his only chance of getting a plan through the Senate is with a simple majority. That leaves him with the "reconciliation process," which only lets him touch parts of the law that affect the federal budget — not, say, the entire health care system.

"What people are learning is, this reconciliation tool is pretty tight."

Alan Frumin understands the reconciliation process probably better than anyone else — and he agrees. Frumin served as Senate parliamentarian for 18 years before retiring to Florida in 2012. He refereed some of the Senate's most contentious health care fights, including the 1994 Clinton reform effort and Obamacare's 2010 passage. We spoke Tuesday afternoon. Here are some of the key points Frumin made:

Reconciliation is not a good process for passing major bills. The reason reconciliation exists is to tweak existing programs — not to create new ones. So it will be difficult for the GOP to shoehorn a large bill into a constricting process.

"The things that make it challenging for a professional parliamentarian is that reconciliation was designed for minor budgetary adjustments, not major policy proposals," Frumin says.

This often put Frumin in the uncomfortable position of telling senators what they couldn't use reconciliation to do. "It's a terrible position to be in, which is why I'm under the palm trees in Florida," he says.

Frumin is skeptical that a key policy in the GOP plan will hew to the reconciliation rules... Frumin has not read the GOP health bill, but he let me describe to him one key proposal: requiring insurers to add a 30 percent surcharge to premiums for those who have a break in coverage. This is important because it's the one provision in the law that encourages healthy people to enroll in coverage — and Frumin wasn't sure if it would pass muster.

"The first question would be what is the score in terms of receipts to the government and the outlays," he said. "This seems to be more a question of payments to private entities, so I'm skeptical there is a legitimate budgetary score."

...but senators and their staff can often make surprisingly convincing arguments. It was notable that Ryan seemed really confident in his press conference that he had drafted a bill that could survive the reconciliation process.

And Frumin was quick to caution that he wouldn't rule anything as not meeting reconciliation rules before hearing arguments from staff. It is completely possible that Republicans could offer a compelling defense of why this provision should be counted under the reconciliation rules.

"We know process, and they know substance," Frumin says. "We have to filter substantive proposals through a sieve. We are duty-bound, once we've told them to make their case, to listen to their arguments."

Read more on the procedural obstacles in the Senate here.

Sign up for VoxCare, edited by Sarah Kliff Vox's daily email explaining the biggest news in health care. By signing up, you agree to our terms. For more newsletters, check out our newsletters page.

Brookings: "At least" 15 million would lose coverage under AHCA

The think tank released a very "whoa if true" report from Loren Adler and Matt Fiedler, who predict that the Congressional Budget Office score of the GOP plan will show at least 15 million people losing coverage. They estimate that the repeal of the individual mandate coupled with big cuts to both the Medicaid and private insurance expansions will cause the massive coverage loss. Read their report here.

There is one other estimate on coverage loss available so far, from S&P Ratings, and it is smaller. The ratings agency thinks 6 million to 10 million people would lose coverage under AHCA.

Still, neither of those numbers is exactly good news for Republicans. The analysts aren't split on whether millions of people will lose coverage. They're split on how many million will.

Chart of the Day

Despite President Trump’s claim that the health care bill is “coming along great,” however, the bill faces much opposition from a slew of major health care lobbying groups, conservative political organizations, right-wing media, and many Republican lawmakers (not to mention all the liberals who have spoken out against it). My colleagues Julia Belluz and Jacob Gardenswartz pulled together this (very long) list of the major groups now opposed.

Kliff's Notes: Today's top 3 health policy reads

With research help from Caitlin Davis