WASHINGTON — With the first enrollment deadline now passed, Republicans who have made the repeal of President Obama’s health care law their central aim are confronting a new reality: More than two million Americans are expected to be getting their health insurance through the Affordable Care Act come Jan. 1.

The enrollment figures may be well short of what the Obama administration had hoped for. But the fact that a significant number of Americans are now benefiting from the program is resulting in a subtle shift among Republicans.

“It’s no longer just a piece of paper that you can repeal and it goes away,” said Senator Ron Johnson, Republican of Wisconsin and a Tea Party favorite. “There’s something there. We have to recognize that reality. We have to deal with the people that are currently covered under Obamacare.”

And that underscores a central fact of American politics since Franklin D. Roosevelt signed the Social Security Act during the Depression: Once a benefit has been bestowed, it is nearly impossible to take it away.