Over the past several weeks, President Obama has repeatedly declared the debate concerning the repeal of his signature health care law "over." And, to be sure, there is a certain rationale to his confidence. For all their talk about shrinking government, Republicans have never had much success rolling back major federal programs, and now that millions of Americans are receiving insurance through Obamacare, there's reason to believe it's here to stay.

That having been said, there are a number of reasons why Obamacare may be more vulnerable than previously enacted entitlements. The two examples usually given to demonstrate how programs can become entrenched are Social Security and Medicare. But there are important differences between those pillars of the modern liberal welfare state and Obamacare.

To start, Social Security and Medicare were more broadly supported at the time they were enacted. Both programs passed Congress with overwhelming bipartisan majorities. Social Security had the support of 68 percent of the population in 1936, a year after its passage, according to research from Michael Schiltz - and that support rose to 77 percent in 1937. In October 1964, the year before Medicare passed, a Gallup poll found 61 percent of public in favor of the idea and just 31 percent opposed.

In contrast, Obamacare passed on a straight party line vote and has remained consistently unpopular from the days it was being debated in 2009 and throughout its implementation. By a more than 11-point margin, Americans oppose the health care law, according to the most recent average of polls compiled by Real Clear Politics.

But beyond this, the reason why Medicare and Social Security have proven so resilient is that the popular perception is that they are not welfare programs. People have bought into the idea that everybody “pays into” the programs as they work, and are merely getting back what they put into the system when they retire (never mind that, overall, benefits far exceed contributions). Additionally, the benefits are broadly distributed – people have parents or grandparents who are benefitting from the programs and they know that at one point they, too, will be old. The benefits flow to both wealthy and low-income individuals.

Obamacare functions much differently. According to projections by the Congressional Budget Office, Obamacare will spend roughly $2 trillion to extend insurance to just 6 percent of the population of legal U.S. residents. That's a rather large bill for a small percentage of the population relative to the number of people who benefit from Medicare or Social Security.

Republicans were never able to seriously reform Medicare or Social Security, but they were able to reform welfare – in a Democratic administration. This isn’t to say it will be easy for the GOP to repeal Obamacare, but the program’s future may be a lot more precarious than the president is letting on.