WASHINGTON — With the House poised to vote Friday on yet another bill to cripple President Obama’s health care law, the question arises: Why do Republicans persist in their so-far futile efforts?

Democrats have many theories. Republicans, they suggest, care little about the uninsured. Many, they say, dislike Mr. Obama and want him to fail.

“The health care law has become the Republicans’ great white whale,” Representative Henry A. Waxman, Democrat of California, said Thursday. “They will stop at nothing to kill it.”

Republicans say they persist because the law is an example of government overreach and is proving unworkable. For many elected in the 2010 Republican wave, their opposition to the law is the reason they are in Congress. And, they say, voting to repeal the law is good politics, as it remains extremely unpopular among Republican voters.