Sen. Robert C. Smith (R-N.H.) plans to bolt the Republican Party and continue his candidacy for the presidency under the banner of the U.S. Taxpayers Party, sources said today, infuriating party officials meeting here.

Republican leaders considered taking steps to prevent any other potential dissident among the presidential candidates from following Smith's lead by requiring a loyalty pledge. But tonight it remained unclear whether they would follow through with their threat.

Smith met privately with his staff in New Hampshire to tell them he has decided to run for president on the Taxpayers Party ballot, a third party headed by conservative Howard Phillips, according to sources. Smith is scheduled to discuss his plans on the "Larry King Live" TV show Monday night and formally announce his decision in a Senate speech on Tuesday.

The two-term New Hampshire senator has been increasingly critical of his own party, saying many Republicans have abandoned conservatives by softening their positions on abortion and gun control. He has publicly threatened to leave the GOP.

Members of the Republican National Committee meeting here ridiculed Smith's decision as the sour grape of a candidate whose bid for the nomination has remained frozen with only 1 to 2 percent support in polls, even in his home state.

"Your 1 percent standing in New Hampshire doesn't reflect Republican primary voters' rejection of your message, but rather its redundancy," RNC Chairman Jim Nicholson wrote Smith. "I hope you don't confuse the success of our shared message with your failure as its messenger."

In the Senate, Smith could be stripped of his membership in the party caucus and lose his committee seniority. Smith is chairman of the Senate Ethics Committee and in line to be chairman of the environment committee. A spokesman for Senate Majority Leader Trent Lott (R-Miss.) would only say that the full GOP caucus will meet with Smith. Meanwhile, Sen. Mitch McConnell (R-Ky.), chairman of the National Republican Senatorial Committee, geared up to demand the return of the hundreds of thousands of dollars the committee gave Smith in his tough reelection fight in 1996, according to GOP sources.

At the RNC meeting, two key party chairmen, acutely aware of the damage done to the GOP by the third-party bids of Ross Perot in 1992 and 1996, said that they supported calling on the remaining GOP presidential candidates to sign a "12th Commandment" pledge to support the Republican nominee in 2000 and to reject all proposals to run a third-party or independent bid.

The "12th Commandment" pledge -- the 11th is Ronald Reagan's "Thou shalt not criticize a fellow Republican" -- was proposed by Stephen Duprey, the party's chairman in New Hampshire, which holds the first primary in 2000, and by Kayne Robinson, chairman in Iowa, which holds the first caucus.

Two conservative candidates clearly were unenthusiastic about the idea. "I think this pledge is a bad idea," Gary Bauer said in an interview. "If we abandon the [conservative] Reagan vision, I just don't think pledges will mean very much." A spokesman for Patrick J. Buchanan said the candidate would not comment until he sees the proposed pledge in written form. Buchanan supporters have been pressing him to seek the nomination of Perot's Reform Party.

Republicans here said a Smith candidacy on the Taxpayer Party ticket, which will not have a ballot line in many states, will inflict minimal damage on the GOP nominee, although in a tight race a tiny percentage loss can prove fatal. GOP leaders are far more concerned that a conservative will defect and seek the Reform Party nomination, which will be on the ballot in many states.

Duprey contended that the pledge request, which was dropped or at the very least delayed after a closed-door meeting of RNC members, is a fair demand: "If you are going to run in our primary, seek our party's nomination and use our party's resources, then you owe us your support and loyalty for the nominee of our choice. I have no tolerance for people who run in our primary and then leave."

Robinson and Duprey said they cannot bar candidates from participating in party events, but they warned that they intend to publicize whether candidates are willing to sign the pledge. "We'll ask them. And they'll be unpopular if they give the wrong answer," Robinson said.

Duprey was more blunt. "I am hard-nosed," he said. "I consider it a benchmark. For me there is no gray about this."