This undated photo provided the office of U.S. Senator Rob Portman shows Sen. Portman, right, and his son, Will. Sen. Portman is now supporting gay marriage and says his reversal on the issue began when he learned Will is gay. (AP Photo/Office of U.S. Sen. Rob Portman) (Uncredited/AP)

As the U.S. Supreme Court is gearing up to hear oral arguments on two prominent cases related to same-sex marriage, prominent Republican Senator Rob Portman’s (Ohio) surprise announcement that he now supports allowing gay and lesbian couples to marry has created a media frenzy and sent conservative commentators scrambling. While Portman’s reasons were extremely personal-- his relationship with his son, who is gay—they cast light on the measurable power of personal relationships on political views. Even on controversial issues such as same-sex marriage, public opinion research demonstrates that relationships with family and friends often trump political and religious affiliations.

Since 2011, polls have consistently shown pluralities or slim majorities of Americans—including many religious Americans such as Sen. Portman’s fellow mainline Protestants—have come to support allowing gay and lesbian people to marry legally. It is also well known that the polls also show as much as a forty-point generation gap between the youngest and oldest American adults on this issue.

But what is less known is what might be called the “friends and family effect”: the nearly 30-point gap that exists in attitudes on same-sex marriage between those who have a close friend or family member who is gay or lesbian, and those who do not. Among the approximately 58 percent of Americans who report having a close friend or family member who is gay or lesbian, six-in-10 (60 percent) support allowing gay or lesbian couples to marry, compared to 35 percent who oppose it. Among Americans who do not have a close friend who is gay or lesbian, opinions are a mirror image: only three-in-10 (31 percent) support same-sex marriage, and 62 percent oppose it.

The gap due to this friends and family effect exists in virtually every demographic subgroup; it is 36 points among women, 23 points among men, 28 points among whites, 30 points among those with a college degree, 27 points among those will less than a college education, and 19 points among white evangelical Protestants.

One exception to this general pattern—and one reason why we may have seen few Republican leaders speaking out as Portman has—is that is that the friends and family effect is considerably more muted among Republicans. The friends and family gap among Republicans, who are much more opposed to same-sex marriage and significantly less likely than other Americans to report having a close friend or family member who is gay or lesbian, is only 10 points, compared to 23 points among independents and 39 points among Democrats.

Even though the friends and family effect is less pronounced among Republicans, Portman is not the first GOP leader to publicly connect a change of mind with a family bond. In a number of interviews Sen. Portman has given in the wake of his announcement, he described getting the counsel of former Vice President Dick Cheney, one of the few other prominent Republicans who publicly bucked his party’s position; Cheney has also cited his relationship with his lesbian daughter Mary Cheney as a motivation for his position. In a quote that puts a human face on the friends and family effect in the numbers, Portman reported that Cheney’s advice was, “Do the right thing. Follow your heart.” If Sen. Portman is in the minority among his fellow Republicans, he is in good company among the many Americans whose views are impacted by their close relationships with gay and lesbian friends and family members, and who have chosen to follow hearts over politics on this issue.

Note: Analysis above based on results from the PRRI Religion and Politics Tracking Poll, August 2011 (N=1,006). The poll was designed and conducted by Public Religion Research Institute and has a margin of error of +/- 3.0 percentage points at the 95% level of confidence. Full results can be viewed here.