WASHINGTON — Next to the oath of office, it has been perhaps the most important commitment Republicans in Congress can make. It is called simply “the Pledge,” and its enforcer is such a fixture in the party that he is known simply by his first name, Grover.

Signing it means a promise to never, ever vote for a tax increase.

But the pledge and its creator, Grover Norquist, a 56-year-old conservative lobbyist, have never before faced a test as they do now. The federal deficit stands at $1 trillion. The social safety net continues to grow — and, in the case of Medicare and Social Security, remains hugely popular. And unless the two parties can agree on a fiscal plan before Jan. 1, hundreds of billions of dollars of tax increases will go into effect automatically, meaning that Congress does not even need to act for taxes to rise.

The combination means that Norquist, whose long record of success is a rarity in Washington, finds himself in a tricky spot. Some top Republicans, including House Speaker John A. Boehner, are saying they now agree with Democrats that the government must collect more tax revenue. Others have gone so far as to break with Norquist publicly.

By Norquist’s count, 219 House members — enough for a majority — and 39 senators have committed to the pledge. But some of those members who signed on, many of them years ago, have started to distance themselves, apparently leaving him several votes shy of the majority he would need to block any tax increase.

“A pledge is good at the time you sign it,” said Rep. Peter T. King, a Republican from Long Island whose name still appears as a pledge signer on the website of Norquist’s group, Americans for Tax Reform. “In 1941, I would have voted to declare war on Japan. But each Congress is a new Congress. And I don’t think you can have a rule that you’re never going to raise taxes or that you’re never going to lower taxes. I don’t want to rule anything out.”

Sen. John McCain, R-Ariz., noted with a certain sense of satisfaction at an Atlantic magazine forum last week that “Fewer and fewer people are signing this, quote, ‘pledge.’ ”

Sen. Lindsey Graham of South Carolina, at the same forum, said that with the federal debt at $16 trillion, closing tax loopholes and eliminating deductions have to be considered “even though that may technically violate the pledge.” He added, “Sign me up.”

Ted Yoho, who will represent part of northern Florida when new House convenes in January, said: “I’ll pledge allegiance to the flag. I’ll pledge to be faithful to my wife.” But he is one of the House newcomers who declined to sign to Norquist’s pledge and likened it to a New Year’s resolution many others will break.

Norquist claims to have invented the idea for a no-new-taxes promise when he was a 12-year-old volunteer for Richard Nixon’s 1968 presidential campaign.

He is fond of evocative metaphors, like claiming he wants to reduce government “down to the size where we can drown it in the bathtub.” Not honoring the pledge would cause grave damage to the Republican brand, he has said, likening it to a Coke can with a rat head in the bottom.

The walls at the headquarters of his interest group are covered with signed copies from conservative heroes like Newt Gingrich, who warns Republicans to stick to their guns. Gingrich, like Norquist, argues that the pledge protects Republicans from agreeing to stealth tax increases that ultimately hurt them.

“Every time I’ve watched Republicans try to be clever with Democrats on issues of taxation,” Gingrich said last week, “Democrats have won.”

In the current talks with President Barack Obama, Republicans have signaled an openness to increasing tax revenue by reducing deductions and credits, as long as income tax rates do not rise. That would still violate the pledge, which states that “any and all efforts” to increase taxes are inexcusable.

Some Republicans have decided they no longer like the lack of flexibility. “Basically the pledge is like a Master Lock,” said Rep. Scott Rigell of Virginia, who in January became one of the first freshmen to publicly renounce the pledge.

When Rigell announced his reversal, he braced for blowback. Instead, he said the word he received was a text message from his local Republican Party chairman saying, “Thanks for your courage.” Yet Norquist still includes Rigell’s name on his list of pledge signers.

Obama administration officials and other Democrats argue that Norquist’s pledge is doomed because Republicans will eventually need to square their budget policies with public opinion. With society aging and modern medicine developing new treatments, health care costs will continue to rise.