The newfound political magic of George Bush has spread so persistently since his Iowa triumph that Ronald Reagan's managers privately conclude they face disaster in the Feb. 26 New Hampshire primary unless Bush is effectively portrayed as an upper-class front man for the old Nixon gang.

The question is: who will do it? Reagan himself believes in the 11th commandment, forbidding one Republican to speak ill of another. William Loeb's Manchester Union-Leader onslaught on Bush evokes yawns. Sen. Howard Baker has begun to assault Bush but pulls punches after 13 yards of senatorial politesse.

If nobody draws blood before then, Reagan must overpower Bush in the Feb. 23 two-man debate his managers engineered. That is his last chance to avoid a defeat here that would make Bush the commanding favorite for the Republican nomination.

The conventional wisdom that Bush and Reagan are neck-and-neck is distrusted by neutral Republican politicians, who suspect steady leakage of Reagan voters. The reason: George Bush is a political phenomenon. While taking conventional conservative positions in conventional Republican prose, his enthusiasm infects his overflow audiences.

The rapport was obvious one day last week at a Lebanon town hall and even more intense that night before a Dartmount College town-and-gown crowd of 1,500. When his call for a strengthened CIA produced the evening's most protracted applause, it showed how quickly students have changed. But more than rising conservation, Bush gains from coming across as the happiest warrior since Hubert Humphrey.

Reagan's first post-Iowa response was to point up his superior conservative credentials on opposing the Equal Rights Amendment, abortion and gun control. This and ringing rhetoric on pro-defense, anti-Soviet policies titillated Reagan's own hard core. But it did not stem Bush's tide.

Rep. Jack Kemp, Reagan's economic spokesman, insists that the answer is to emphasize economic questions. While Reagan has embraced the Kemp-Roth tax cut, Bush distrusts current Republican tax-reduction philosophy.

Reagan, sounds more like Kemp than Reagan on new 30-second television spots written for him by Jeff Bell, New Jersey Senate candidate in 1978 and a Kemp acolyte. But how comfortable Reagan is in this stance is doubtful. Campaigning in New Hampshire last week, he fell back on relating details of his triumphs as governor of California. In a 15-minute luncheon speech in Merrimack, he devoted 15 seconds to tax reduction.

Nor is Jerry Carmen, the rough-and-tumble Manchester Republican who runs Reagan's campaign, enamored with Kemp's economics. He had campaign staffers searching the record for anti-Bush material. They found a tiny nugget in Jules Witcover's "Marathon," a 1976 campaign book: "Everyone knowledgable in Republican politics considered Bush incompetent to be president."

Carmen wants to brand Bush as a country-club candidate whose backers went to Ivy League colleges, worked in the CIA and joined the Trilateral Commission. Most of all, he wants identification of Bush with Richard M. Nixon. Reagan operatives have tried to keep alive the decade-old story of Nixon "slush fund" contributions to him, but nobody except the Union-Leader pays much attention (and one Reagan insider confides: "I'm afraid Bill Loeb just lacks credibility").

But Howard Baker doesn't. The Reagan camp has nearly abandoned hope that Baker's late-blooming campaign will take-away many Bush votes, but it wants him to do what Reagan won't: cut him down. Baker last week by pointing to Bush's two statewide defeats in Texas, disagreeing with Bush over revenue sharing, suggesting Bush is overly hawkish. But criticism of "my good friend George Bush" was too steeped in senatorial circumlocution to suit Jerry Carmen.

Carmen wants more of what occurred at the end of a lackluster Baker rally in Concord. Fred S. Parker, an insurance man who runs Baker's campaign in Keene and contends that Bush is tainted with Watergate, rose with a leading question asking Baker to compare himself with Bush. Baker concluded his answer by noting his disapproval of Gerald Ford's pardon of Nixon. "I expect that George and I disagree on that," he added, pointing out that Bush was Nixon's hand-picked Republican national chairman.

Characteristically, Baker quickly retreated to a pointless anecdote about Nixon, after all, being a human being. Baker is ill-cast as George Bush's destroyer. That improbable role must fall to Reagan, in the all-candidate debate Feb. 20 and the two-man confrontation Feb. 23 sought by Carmen. For Ronald Reagan at age 69, it is perhaps his hardest challenge in 14 years of elective politics.