Senate Majority Leader Mitch McConnell, R-Ky., followed by Sen. John Barrasso, R-Wyo., and Sen. Cory Gardner, R-Colo. Bill Clark | CQ Roll Call | Getty Images

Why the Senate will pass their health care bill:

1) McConnell appeases the moderates on Medicaid and insurance regulations. The more moderate Republicans in the Senate had some big problems with the House bill, which rolled back some of Obamacare's insurance protections, its financial aid to help people purchase private insurance, and overhauled Medicaid, the nation's biggest insurer. They thought the Medicaid cuts, including an abrupt end to Obamacare's Medicaid expansion in 2020, were too harsh and they opposed rolling back the law's protections for people with preexisting medical conditions. McConnell has moved in their direction on those issues, proposing a more gradual three-year phaseout of the Medicaid expansion and keeping the Obamacare rule that prohibits health plans from charging sick people more than healthy people. In turn, some of the Senate moderates thought to be most difficult for McConnell to get have said positive things about the emerging plan. "It's clear from the outline that I have seen of the Senate's — where there were a lot of question marks, so they'll be filled in — it's quite different from the House bill, and I think that's a good thing," Sen. Susan Collins (R-ME) told reporters last week. If McConnell wins over all the moderates, then he can lose the most conservative senators — Rand Paul of Kentucky and Mike Lee of Utah — and still pass the bill. 2) He threads the needle with enough moderates while holding the right flank. McConnell could also go in the opposite direction and sacrifice two moderate votes, likely to be Collins and perhaps Sen. Lisa Murkowski (R-AK), who told me last week that she was very frustrated with the House bill and the Senate's secretive process. In that case, he would have to win over the conservatives. The Hill and Axios reported Monday that the Senate could end up including even lower Medicaid spending caps than the House bill did, a provision that could help appease the right flank. Though they tend to be more ideological than the moderates, conservatives can be practical too. "If you told me I couldn't repeal everything and some of Obamacare would remain, I would vote for that as an imperfect bill," Paul told me last week. McConnell could keep a few of the moderates worried about Medicaid on board by offering additional funding for the opioid crisis — Sens. Rob Portman (R-OH) and Shelley Moore Capito (R-WV) are asking for $45 billion over the next 10 years — and still get to 50 votes without Collins or Murkowski. 3) They're Republicans and it's a Republican health care bill, stupid. The power of partisanship helped the House move from disastrously short on votes to narrowly passing a bill over the course of one month. The same dynamic could push senators to get in line; McConnell told them privately that they would be paving the way for single-payer health care if they didn't pass this bill. Take Sen. Dean Heller (R-NV), who faces the most difficult reelection campaign next year. He represents an increasingly blue state that expanded Medicaid and where the Republican governor has criticized the House bill. That's a lot of reasons to vote against any similar plan. Nevertheless, when I asked Jon Ralston, editor of the Nevada Independent and the dean of the state's political reporters, if Heller could still end up supporting the bill, he cited partisanship as one reason Heller might come around (emphasis mine): Politics of next year say he should vote against it because of the polling here that shows how unpopular it is. Then again,hemight lose the base if he votes no, which has to concern him. Dilemma for many Republicans, but especially for him because he is so vulnerable. Seth Masket, a political science professor at the University of Denver, made a similar point when I asked him why Sen. Cory Gardner (R-CO), another swing vote from a swing state, might vote for the bill. "Voting for the health care bill could be damaging to him in his next reelection race," he said, "but voting to kill it could hurt him in the primary and help draw a challenger. So while many senators have grumbled about the GOP's plan, when push comes to shove, it might prove too hard for them to vote against Obamacare repeal and they'll set their concerns aside to back the bill.

Why the Senate won't pass their health care bill: