Julian Zelizer, a history and public affairs professor at Princeton University and a CNN political analyst, is the author of " The Fierce Urgency of Now: Lyndon Johnson, Congress, and the Battle for the Great Society ." He's co-host of the "Politics & Polls" podcast. The opinions expressed in this commentary are his own.

(CNN) In 1965, 13 Senate and 70 House Republicans joined Democrats to pass the Social Security Amendments that created Medicare and Medicaid. Many Republicans in the House and Senate took this vote kicking and screaming, but in the end, they joined the Democratic majority to create the largest expansion of government health care coverage in American history until the Affordable Care Act.

Thursday, after deliberating in the dark of Senate office buildings, Republicans in the upper chamber, led by Mitch McConnell , signaled they will join their House counterparts in dismantling the Affordable Care Act -- thereby stripping away health care coverage for millions of Americans. This will not be an easy vote for the 20 Republican senators whose states have expanded Medicaid under the ACA, and where the changes brought about by the program are quite popular. There are nine Republican senators running for re-election in 2020 whose states have expanded the program. McConnell is telling these Republicans to fall on their partisan sword.

Although the Senate version of the bill will offer some slight modifications to soften the blow, such as keeping subsidies for individual coverage of low-income Americans, overall it looks similar and, in some respects, it is even tougher. It cuts deeply into Medicaid coverage, eliminates crucial taxes and gets rid of the vital individual mandate essential to making this insurance system work and keeping it affordable.

The Senate version of repealing the Affordable Care Act is a powerful reminder of just how much the Republican Party has changed since the 1960s as legislators in the House and Senate have shifted far to the right.

Republicans were divided in 1960s

To be sure, back in 1965 there were many conservative Republicans who opposed Medicare. Ronald Reagan spun a record that warned that Presidents Kennedy and Johnson's Medicare proposal, which provided hospital insurance to the elderly paid for by Social Security taxes, was an opening wedge to socialism.

Sen. Barry Goldwater made his opposition to Medicare a key theme in his presidential campaign against Johnson. "Having given our pensioners their medical care in kind, why not food baskets, why not public housing accommodations, why not vacation resorts, why not a ration of cigarettes for those who smoke and beer for those who drink?" Goldwater asked.

Republicans only proposed their alternatives to Medicare after the landslide Democratic victories in 1964 made it inevitable that some version of the bill was going to pass. Even in the final vote on the conference committee report, 68 House and 17 Senate Republicans voted against the legislation.

But other Republicans made a different choice. Faced with the fact that elderly Americans struggled to afford basic health insurance coverage, liberal Republicans such as New York's Jacob Javits had been a driving force in the push for federal health care coverage.

While many Republicans opposed the administration's Medicare proposal, they did support an alternative in the House, sponsored by ranking Ways and Means Committee Republican John Byrnes, that provided insurance to cover the cost of doctors supported by government funds and contributions from beneficiaries. Many experts believed this plan was actually more generous than the administration's. Even the most conservative wing of the Republican Party pushed for health care for the poor, administered by the states in cooperation with the federal government, which today we know of as Medicaid.

Liberals and moderates gone

British novelist L.P. Hartley wrote, " The past is a foreign country ," and that seems especially true of the Republican Party in the 1960s, given its transformation to what we see today. Liberals and moderates have vanished from the GOP conferences in the House and Senate. Conservative Republicans slowly took over most of the key leadership positions and committee chairs, and there have been fewer and fewer dissenting votes on roll call votes that lean to the right.

In 2010, the tea party was born and institutionalized this rightward drift among Republicans in the House. Members aligned with the tea party formed the Freedom Caucus in 2015, and they have been a force with which to reckon. Under the leadership of Jim Jordan and Mark Meadows, they have created a powerful organizational mechanism within the House that refuses to budge on key issues and that resists the pressure to compromise on core principles. The caucus is willing to go to extremes to pursue its objectives (threatening, for instance, to send the nation into default over disputes about spending) and it has taken down leaders, such as Speaker John Boehner, who opposed it.

For all the talk about McConnell being the master legislator, it looks like his ultimate solution with health care has been to concede to the House plan, tinkering around the edges rather than doing anything to really satisfy the concerns of the Republicans from states where the Medicaid expansion under the ACA and regulations such as the coverage of pre-existing problems remain extremely popular.

After emerging from their secret meetings, McConnell is giving Republicans a stark choice -- vote for the draconian changes passed by the House or get nothing at all. McConnell's decision is evidence that the Freedom Caucus rules the roost on Capitol Hill.

The political problem for Republicans is that the current versions of the health care reform are incredibly unpopular, according to almost every poll. Once the possible effects of the bill are felt, it is likely the anger toward the legislation could intensify, causing serious problems in the 2018 midterm elections and in the next presidential election.

A vote in favor of this health care bill could easily alienate Republican voters who worry about the extremism in the party, and the chaos in the Oval Office, but have remained loyal to their party. Whether Democrats can take advantage of the fallout, that's a different question, one that remains unclear after this week's special election in Georgia . And it is also possible that other issues such as the economy will overwhelm the anger about the health care vote.

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The House and Senate versions of health care reform reveal a Republican Party that is willing to go very far to gut the legacies of the New Deal and Great Society as well as President Barack Obama's domestic agenda. This will be a defining vote for the GOP that puts the party on record for where it stands on protecting Americans from the vicissitudes of health care markets.

For all the attention paid to Trump, his tweets and the controversy over Russia, the reality is that in today's Washington the real story is the power of the Freedom Caucus.