President Bill Clinton had just been ambushed by his own party. It was August 1994 and a coalition of House Democrats wary of any new gun restrictions joined Republicans to unexpectedly sink the administration’s big crime bill on a procedural vote that was usually a test of party loyalty.

Afterward, Speaker Thomas S. Foley and his top lieutenants, all Democrats, trooped down to the White House with a message for a shocked president who was already struggling on his signature health care proposal: Drop a divisive ban on assault weapons or the crime bill won’t pass.

“To his credit, Clinton said, ‘No, we are not going to do that,’” recalled Rahm Emanuel, who was then a senior policy adviser to the president and was at the Green Room get-together with Mr. Clinton and House leaders. “That’s when we decided to go to the Republicans. At the time, it was novel to try to work with the Republicans.”

With Congress prepared to again clash over gun safety, in the aftermath of a murderous August, the circuitous route to passage taken by the assault weapons ban 25 years ago illustrates just how perfectly the legislative stars must align for contentious gun measures to become law. It also shows what such an effort entails — true bipartisanship, a committed White House, a readiness on all sides to compromise and a willingness by some lawmakers to take a significant political risk.