A friend and I have an ongoing debate about the reasons for the plague of partisan rancor now afflicting Washington in general and arms control in particular. For my friend, the passage of the Affordable Care Act (aka “Obamacare”) was the Rubicon. Before Obamacare, he points out that important domestic legislation received bipartisan support. These numbers back up his argument:

Social Security Act of 1935
60 Democrats yes; 16 Republicans yes
1 Democrats no; 5 Republicans no
284 Democrats yes; 81 Republicans yes
15 Democrats no; 15 Republicans no

Civil Rights Act of 1964
46 Democrats yes; 27 Republicans yes
21 Democrats no; 6 Republicans no
152 Democrats yes; 138 Republicans yes
96 Democrats no; 34 Republicans no

Affordable Care Act
58 Democrats yes; 2 Independents yes; 0 Republicans yes
0 Democrats no; 39 Republicans no
219 Democrats yes; 0 Republicans yes
34 Democrats no; 178 Republicans no

After the White House and the Democratic leadership on Capitol Hill rammed through Obamacare, my friend believes that Republican Members of Congress resolved not to work with President Obama. In my view, the absence of bipartisanship predates the battles over health care, reflecting quarter-century-long trends within the Republican Party and deepening divisions within the country at large.

Here’s my reasoning: Trends toward assured incumbency, reinforced by two decades of redistricting and gerrymandering prior to Obamacare, have led to sharper partisanship. When seats are assured to one party or the other, the locus of competition shifts from general elections to primaries, and cross-over voting on Capitol Hill becomes rarer. Bipartisan votes were a regular occurrence on motherhood-and-apple-pie issues. They are now restricted to apple pie. The “do nothing” Congress that Harry S Truman used as a foil to win the presidency in 1948 passed 906 bills. Compare that with what passes for legislating at present.

Much political commentary dwells on the defeat of Tea Party candidates as an indicator of the supremacy of Main Street Republicanism, even though every serious challenge from the Right – regardless of the outcome — reinforces the Tea Party agenda on Capitol Hill. The most dramatic Tea Party upset win — the defeat of Minority Leader Eric Cantor, who was traveling the country playing out ambitions to become Speaker instead of stumping his district — was attributed to his openness to immigration reform legislation. Immigration legislation is now dead for the foreseeable future. An analysis in the New York Times of genial, low-key Senator Thad Cochran’s narrow Senate primary victory over a Tea Party challenger in Mississippi offered a cautionary note to Republicans: “It is no longer enough to quietly represent your constituents. You have to join the partisan fray.”

This kind of partisanship doesn’t stop at the water’s edge. The internationally-minded wing of the Republican Party in the Senate has been decimated since the George H.W. Bush administration negotiated two strategic arms reduction treaties with Russia. Its remaining spokespersons, John McCain and Lindsey Graham, lost their standing by championing wars prosecuted by the George W. Bush administration that were poorly conceived and badly executed.

If the reluctant Jeb Bush declines to run for President in 2016, the current crop of Republican contenders will be headlined by (1) Ted Cruz, whose principal achievements in the Senate to date are temporarily closing down the government and blocking ratification of a Treaty recognizing the rights of the disabled; (2) Rand Paul, the Republican analog to George McGovern; and (3) Marco Rubio, whose impact on the Senate has been felt mostly by placing holds on the Obama administration’s nominees. What they have in common, besides opposition to arms-control treaties as an infringement of U.S. sovereignty and military capabilities, is the resolute pursuit of deficit reduction at a time of declining U.S. influence in the world.

The biggest Democratic makeover during the past quarter-century was when Bill Clinton moved his Party toward the center. The Republican Party’s biggest makeover has been to move away from the political center. Self-professed Reagan Republicans like Cruz actually have little in common with Reagan’s record of legislation in Sacramento and Washington. One example: Reagan wanted to eliminate nuclear weapons. Republicans on Capitol Hill now want to stop dismantling empty missile silos.

When did the Republican Party lose its moorings? Barack Obama’s presidency and his pursuit of the Affordable Care Act were certainly accentuating factors, but the shift away from the legacies of Presidents Eisenhower, Nixon, Ford, Reagan and George H.W. Bush started earlier.

In retrospect, the pivot point might have been George H.W. Bush’s defeat by Bill Clinton in 1992. Four years later, Fox News was launched, broadcasting its daily dose of fear and loathing. Then came the Clinton impeachment circus in 1998. What clearer indicator can there be of going off-kilter than to elevate a sexual indiscretion to the level of impeachment proceedings? Whitewater has been followed by Benghazi, with stops in between during every news cycle. There’s more to come, as Hillary gears up for another presidential run.

What does all this mean for arms control? The first rule of pursuing treaty ratification is to avoid partisanship. Democrats in the Oval Office will have a harder time doing this than Republicans, but it’s unclear when the next Republican President will be sworn in, since the Republican Party has adopted political agendas that work far better in safe Congressional districts than nationally. Meanwhile, Republicans on Capitol Hill are chipping away at the treaties Ronald Reagan and George H.W. Bush once championed. What mainstream Republicans used to consider agreements that advanced U.S. national security are now considered infringements on America’s freedom of action.