Many Democrats are genuinely puzzled about Republicans' continuing opposition to Obamacare. It is the law of the land, these Democrats say. Critics should accept it, as critics accepted Medicare.

They should work constructively and across the aisle with Democrats to repair any flaws and make the law work to help people.

Historical analogies are often useful, but can be misleading. Certainly so in this case: Republicans, like it or not, are behaving differently from the way they behaved after the passage of Medicare in 1965.

To understand why there is continued resistance to Obamacare and why majorities of voters continue to oppose it in polls, a different historical analogy is helpful.

It is an example of a law that was bitterly opposed but that was accepted by opponents to a much greater extent than even many of its advocates expected: the Civil Rights Act of 1964.

The most controversial provision of the law was Title II, prohibiting racial discrimination in public accommodations -- hotels, motels, restaurants and theaters. This overturned Southern state laws requiring racial segregation in such facilities.

There was good reason to believe that this law would be hard to enforce in practice, as recent experience of the Freedom Riders showed.

Starting in May 1961, civil rights groups organized biracial groups to ride on interstate bus lines in the South. Segregated interstate transportation had been ruled unconstitutional by the Supreme Court in 1946, but Southern states ignored the ruling.

Freedom Riders were physically attacked with baseball bats and bicycle chains in South Carolina, Alabama and Mississippi. Birmingham police chief Bull Connor (then a Democratic National Committeeman) organized mob attacks. A bus was firebombed near Anniston, Ala.

Attorney General Robert Kennedy called for a "cooling off period." The Kennedy administration eventually got Southern governors to provide police escorts for Freedom Ride buses and not to interfere if the Riders were arrested.

I have often wondered what the politicians and journalists who favored equal rights but urged civil rights protesters to go slowly were thinking. They must have believed that protests would provoke violence, and that most of the people hurt would be black.

Surely many of those who supported desegregating public accommodations must have feared widespread noncompliance and continuing violence. But in fact these things did not happen to any great extent.

Why not? The way the law was passed.

In May 1963, Bull Connor turned police dogs and fire hoses on protesters in Birmingham. Technological developments enabled evening newscasts to bring the Birmingham story into Americans' living rooms.

A feeling that this was intolerable and that something must be done swept most of the nation. In June, President Kennedy went on evening television and endorsed a civil rights bill including public accommodations.

Congress had passed, after much deliberation, civil rights laws of narrower scope and lesser effect in 1957 and 1960. It proceeded with careful deliberation to pass a stronger bill this time.

The House Judiciary Committee reported a bill in November, just before Kennedy's assassination. The chairman of the Rules Committee, Howard Smith of Virginia, said he would not allow it to be considered.

Supporters sought the signatures of a majority of House members needed to bring the bill to the floor. They got them after members heard from their constituents over the winter break. The bill went to the floor in February and passed with bipartisan support, 290-130.

In the Senate, Southerners launched a filibuster that lasted 57 working days. It then required 67 votes to cut off debate. But in June, 71 senators voted for cloture and the bill passed 73-27.

Full compliance with the public accommodations section was not immediate. In Georgia, Lester Maddox closed his restaurant rather than serve blacks, and then was elected governor, narrowly, in 1966.

But after Congress acted in such deliberate fashion, and the Supreme Court upheld the law, white Southerners largely acquiesced. Traditional Southern courtesy replaced mob violence. Minds and hearts had been changed.

Obamacare has been a different story. Universal health care was promised, not to address a high-profile headline crisis, but because President Obama's twenty-something speechwriter wanted an applause line for a campaign speech.

The poorly drafted bill was passed almost entirely on party lines by exceedingly narrow margins -- and in the face of majority negative public opinion.

So it's not surprising that opponents won't accept its legitimacy or permanence. History tells us what that takes.