The right-wing coalition of Southern whites and Bible Belt social conservatives, which Richard M. Nixon cobbled together in the late 1960s and dominated American politics for nearly a half-century, has run its course.

Its power to shape national policy reached its peak under Ronald Reagan. Bill Clinton's election in 1992 slowed the rightward swing in American politics. Twenty-first century America is moving to the political center, making the present-day Republican Party a mostly-regional, special interest party. That narrow right-wing coalition lacks the political strength or public support to pass its agenda. Its only power is to obstruct, and it isn't shy about using it. Gay marriage and the national debate on gun policy demonstrate this coalition's problems.

A sea change in public opinion has put pressure on state and federal governments to remove bans on same-sex marriage, and while it's risky business predicting Supreme Court decisions, many expect that the court will either uphold the reversal of California's Proposition 8, declare the Defense of Marriage Act unconstitutional, or both. Should that happen, social conservatives will have lost one of their last ballot-box issues.

The epidemic of mass shootings, most recently at Sandy Hook Elementary School, forged a majority opinion among Americans supporting both strong background checks and some form of firearms regulation. A study released last week by Quinnipiac University reported that 91 percent of American voters support expanded background checks for all gun buyers, compared with just 7 percent who say they're opposed.

Sixty-one percent of Americans support reasonable gun control laws, such as reducing the allowable size of ammunition clips. Even allowing for response bias, that majority is impossible to dismiss. President Barack Obama noted this when he said on Monday, "If our democracy is working the way it's supposed to, and 90 percent of the American people agree on something, in the wake of a tragedy you'd think this would not be a heavy lift."

But it is. Almost all opposition to these measures comes from Republicans, and the few anti-regulation Democrats representing states with Republican majorities.

The politics are easy to understand. Studies show that white Southern males, the Republican Party's core constituency, make up the largest group of gun owners.

Republicans are faced with a problem, either support this group and alienate the majority of Americans, or support the majority position and weaken their support within the base.

Earlier this week, 13 Republican senators, 12 of whom come from rural or old confederacy states, swore to block attempts to bring the issues to a vote. They fear being put in a lose-lose situation. If they are forced to vote, then supporting background checks would expose them to primary challenges from extreme right-wing candidates. On the other hand, opposing these popular measures would hurt them in a general election.

Not all Republicans share the senators' fears. To their credit, New York Rep. Peter King and Georgia Sen. Johnny Isakson are among the first Republicans to call for the Senate to hold an up-or-down vote on background checks. As of the time this column was being written, it seemed that more Republicans were moving toward supporting such a vote, which was scheduled for Thursday. Two pro-gun senators, Republican Pat Toomey and Democrat Joe Manchin appear to have come to an agreement on background checks.

NBC News says the compromise will not be as broad as the president had asked. It will probably exempt gun transfers between family members, for example. A compromise on this issue offers some small hope that some of the crazy gun violence may be avoided. It would also give Republicans a chance to move away from being the party of no.