There have been two defining moments in Paul Ryan’s political career: The first was his leading the opposition to Obamacare in 2009 and 2010. (Can anyone forget or on the night of the Obamacare vote in the House?) The second was his getting essentially the entire Republican party to back his crucial, fiscally responsible Medicare reforms (thereby showing it's possible to hold the House and take back the Senate even after courageously touching the "third rail" of American politics).
Donald Trump, so far, has been weak on both issues. He says he wants to repeal Obamacare, but he also says he doesn't want to repeal its onerous preexisting-conditions mandate. That mandate undermines the very notion of insurance--namely, that it must be bought before the thing happens that one is insuring against. It is a core cause of the skyrocketing premiums we've seen since Obamacare was passed. (It is for those with preexisting conditions without undermining the whole notion of insurance, sending prices soaring, and ruining the insurance market.)
Moreover, Trump he says he doesn't want to cut Medicaid, even though it has ballooned under Obamacare. Indeed, most of the coverage increases under Obamacare have come from this huge Medicaid expansion that Trump says he doesn't want to roll back.
Meanwhile, Trump has opposed Ryan's Medicare reforms—probably the most important set of policy proposals in the past decade for trying to rein in our now-nearly $20 trillion in debt ( when President Obama took office).
So a key question coming out of Thursday's meeting between the speaker of the House and the presumptive Republican presidential nominee will be this: Will Ryan be able to emerge from it and tell the American people he has gotten Trump's pledge of support for Ryan's—and the GOP House caucus's—efforts on health-care entitlements? The answer is no small matter. Such an agreement would presumably go a long way toward uniting the Republican party heading into November, while its absence would put much of Ryan's work to date in jeopardy.