WASHINGTON >> The day after the massacre last month in San Bernardino, Calif., Senate Democrats sensing an opportunity rushed to pass a measure denying guns to anyone on the no-fly terrorism watch list. The timing of the vote and the nature of the bill gave them reason to hope that they could, for once, thwart the gun lobby.

But Chris Cox, the National Rifle Association’s brash and boyish-looking chief lobbyist, was ready for them.

Cox and more than a dozen NRA lobbyists under him buttonholed Republican senators in a flurry of face-to-face meetings, calls and emails, officials said. And in a message on Twitter they directed the NRA’s 5 million members to “call your senators NOW and urge them to vote NO on and any all gun control proposals.”

“It was all hands on deck,” David Keene, an NRA board member, said. With the country jittery about guns after 14 people were killed in San Bernardino, he said, “we needed to find out whether these senators were with us or not.”

The outcome was predictable: The Senate rejected the measure preventing those on no-fly lists from buying guns, along with a second one that would have expanded background checks, the latest in a string of legislative victories that have given the NRA a virtual stranglehold on Congress and prompted a frustrated President Barack Obama to issue executive orders to toughen gun restrictions.

At the center of the lobbying blitz, as always, was Cox, 45, a skilled marksman and big-game hunter from Tennessee who has used the NRA’s bully pulpit and his access to Republican leaders to ward off what he says is a backdoor attempt to take away Americans’ guns.

Cox has emerged as the group’s point man in pushing to defeat new gun control laws, expand existing gun rights measures, and gain even more lobbying clout for an organization that he calls “the greatest political force in America.” He has also been instrumental in working on one of the association’s biggest political priorities this year: defeating Hillary Clinton in her bid for the White House.

His boss and mentor, Wayne LaPierre, remains the face of the NRA, a dominating and divisive figure lionized by gun owners and reviled by opponents after nearly four decades with the organization. Cox has a much lower public profile and is sometimes confused with the former congressman and Securities and Exchange Commission chairman with the same name.

But Cox has built up a significant power base of his own in the Capitol, just a few blocks from his office. There, he is known as not only the NRA’s chief lobbyist, but also its kingmaker: He leads its political action committee, which took in $31.3 million in the last three years to dole out on gun rights candidates and causes, and he is the ultimate arbiter of the coveted “grades” the group gives political candidates, which can make or break a campaign.

“You don’t get an A+ grade without Chris’ say-so,” said a former NRA associate, who spoke on the condition of anonymity to discuss the organization’s internal workings.

Under Cox’s supervision, the NRA is known for playing an aggressive role in congressional elections, but it has also been accused of a lack of transparency in its political spending. This month, Citizens for Responsibility and Ethics in Washington, a watchdog group, said the NRA failed to disclose more than $58 million in political spending between 2008 and 2014, and it urged the Internal Revenue Service to investigate.

But it is clear that Cox is highly valued at the NRA After two decades there, he earned $764,000 in salary and bonuses in 2014, second only to LaPierre, its latest filing showed. “He earns it every day,” Keene said.

His visibility also appears to be growing.

When Obama sat for a live town-hall-style event on guns this month, it was Cox who appeared on Fox News before the president had even finished speaking. Cox declared the president’s views “offensive,” “disgusting” and patronizing. And he said he had no interest in accepting an invitation to come to the White House to talk.

“What are we going to talk about — basketball?” he asked. “I’m not really interested in going over and talking to a president who doesn’t have a basic level of respect or understanding of the Second Amendment and law-abiding gun owners in this country.”

Growing up in Tennessee as the son of a surgeon, Cox had no interest in politics.

“I had no idea that this was the path my life would take and that God would guide me down,” he told his hometown newspaper, The Jackson Sun, in an interview last year. (Cox declined requests for an interview for this article.)

But after graduating from Rhodes College in Memphis, Cox went to Washington in the early 1990s for what he thought would be a temporary stint as a legislative aide to his congressman — Rep. John Tanner, a Blue Dog Democrat and a big supporter of gun rights.

“Where we’re from in rural Tennessee,” Tanner said, “the Second Amendment and the gun culture — shotguns, hunting — it’s just part of the DNA, so Chris was part of that culture.”

With President Bill Clinton in the White House, gun control advocates had several big victories, including a landmark ban on assault weapons in 1994. Cox said he took those defeats personally. The next year, he went to work for the biggest name in the gun lobby: the NRA, which began as a sportsmen’s association but grew under LaPierre in the 1980s into a fearsome defender of gun rights.

As head of the NRA, LaPierre, 67, has developed a sharp-edged, battle-ready reputation, and his defiant response after the 2012 slaughter at an elementary school in Newtown, Connecticut — when he declared that “the only thing that stops a bad guy with a gun is a good guy with a gun” — is an iconic moment in the gun debate.

Cox is seen as having a softer edge than his boss, more prone to cajoling than arm-twisting. He knows many senators’ favorite fishing spots, hunting grounds and guns by memory, and he is likely to drop in a reference as he presses his case, associates say.

But while the NRA’s congressional support appears secure, it has faced much stiffer competition at the state level, where gun control groups like Everytown for Gun Safety, funded with $50 million from Michael R. Bloomberg, the former mayor of New York, have scored a number of victories. And when three Virginia state races hinging on gun issues came into play last fall, Cox himself went to the districts to help the pro-gun candidates. (Two of the three NRA-backed candidates won.)

With speeches that can be just as fiery as LaPierre’s, Cox’s frequent attacks on Obama, Hillary Clinton, Vice President Joe Biden and other “dishonest” Democrats always draw raucous applause at NRA events. So do his regular broadsides against the national news media, which he said is “willing to spread lies,” and Bloomberg, a special target.

“I believe God gave us this freedom,” Cox declared at an NRA event. “And we’re not going to let Michael Bloomberg take it away — not now, not ever.”

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