U.S. presidential elections periodically offer a dramatic choice rather than an echo. This was Barry Goldwater’s campaign slogan in 1964 against Lyndon Baines Johnson. Voters chose LBJ in a landslide.  Lop-sided victories are the usual outcome when two starkly different choices face U.S. voters in presidential contests.  Think of  Nixon vs. McGovern, or Hoover vs. FDR. Once every other generation, however, dramatically different presidential choices produce close shaves.  Nixon vs. Humphrey was a nail-biter.  Obama vs. Romney is another.

Back in 1968, voters were told to expect a “new” Nixon. He turned out to be many Nixons: progressive in domestic policies, occasionally brilliant in geopolitical maneuver, just awful regarding the Vietnam War, and paranoid behind closed doors, which cost him his presidency.

There are many Mitt Romneys, too: the progressive Governor of Massachusetts, the right-wing candidate in the Republican primaries, and the moderate who showed up in presidential debates with Mr. Obama. President Obama and his campaign team seemed to have had difficulty figuring out which Mitt Romney to criticize.

If the President is re-elected, we’ll have a pretty good sense of how he will try to tackle very hard problems relating to the Bomb. If Governor Romney is elected, it’s anyone’s guess, given his malleability as a political figure. Clues would be strip-mined from his choice of advisers.  Hard-liners  have kept their counsel while their candidate veered moderate.  For the moment, electoral success matters more to them than conservative orthodoxy.  If Governor Romney wins, Republican factionalism will become fierce, and Democrats will move bitterly to the left.

A tight election result usually does not bode well for the prospects of treaties long held hostage by Republicans in the Senate, whose moderate wing, like the planet’s glaciers, has vastly receded.  Then again, if Governor Romney loses, even by a slim margin in these tough economic times, Republican leaders might just see the wisdom of reconsidering small-tent positions, one of which is to oppose a treaty that confirms the twenty-year-long cessation of nuclear testing by major powers.

The presidential debates have also, in predictable fashion, made it even harder to achieve a diplomatic settlement over Iran’s nuclear program. If reelected, President Obama is likely to make a serious run at a settlement early in his second term.  If Mitt Romney becomes president, he might be capable of leveraging his hard-line position into a surprisingly useful outcome on Iran, just as with Nixon’s opening to China. On Iran, however, the challenger’s position has wavered only marginally, and is more likely to choose combustible outcomes.

Governor Romney’s opposition to New Start constitutes a toxic complement to President Putin’s attachment to ten-warhead, liquid-fueled missiles. Another potentially combustible mix.