When George Bush and Howard H. Baker Jr. (R-Tenn.) appeared back-to-back one night last week before a high school mock convention in North Sutton, a large majority of the reporters left before Baker began to speak.

The exodus was not a sign of personal disrespect for Baker. But it was a reflection of the far greater attention paid to Bush, the presumed Republican front-runner in New Hampshire, than is given to Baker, who is supposed to be a distant third-place candidate.

"That's a good measure of our problem," said a Baker supporter, sadly, as he watched Bush and his traveling press entourage leave the building. "Too many people think this is a two-way race."

Baker faces many obstacles in his effort to demonstrate that the New Hampshire primary and the larger Republican presidential race is more than a contest between Bush and Ronald Reagan.

Baker's crowds have been as scarce as the snow, which today began falling for the first time this winter in New Hampshire. His late-starting organization is believed to be much weaker than the organizations of his major rivals. He lacks the solid conservative base that provides enthusiasm and workers for Reagan. And Baker is second to Bush as a handshaker and backslapper in the plant tours and street walks that are staples of New Hampshire campaigning.

In this context, Baker appears to have been damaged by his exclusion from a Bush-Reagan debate Feb. 23, the Saturday before the primary, Sponsored by The Nashua Telegraph, the state's largest afternoon daily newspaper, the debate has captured the interest of the political community and reinforced the idea that the primary is a two-man race.

"It's arrogant and manipulative for an important newspaper to decide arbitrarily that there are only two candidates before even a single primary has been held," says former New Hampshire governor Walter R. Peterson, the Baker campaign chairman. "But I'm realistic enough to know that an action like this can become a self-fulfilling prophesy."

While being barred from this debate, Baker is being squeezed on his left by the outspoken candidacy of Rep. John B. Anderson (Rep-Ill.), who claims to have climbed into third place.

Anderson pollster Dick Bennett said that a new survey of 386 "likely voters" taken Feb. 8-11 showed Anderson with 11 percentage points to Baker's 8. The other figures, rounded off, were Bush 30, Reagan 29, John B. Connally 3, Philip Crane 1, Bob Dole 1 and undecided 18.

These figures were disputed by Baker cochairman John Michaels, who said Bennett's polls have proved "unreliable" in the past. Without benefit of his survey, Michaels expressed the conventional political wisdom that Baker is running third in New Hampshire, as he did in the Iowa GOP caucuses.

On the stump, Baker is trying to be the thoughtful voter's candidate with a speech more intellictual and discursive than the campaign speeches of Bush or Reagan. His speeches are less partisan and far more charitable to President Carter than the speeches of his rivals.

Last week, Baker tried to sharpen the campaign dialogue by criticizing Bush for opposing revenue-sharing and by making veiled references to Bush's statement, in a recent Los Angeles Times inerview, that there could be a winner in a nuclear war.

"No American president should ever entertain the idea that nuclear war is an acceptable alternative, because there are no winners in a nuclear war," Baker said.

It is a safe presumption that many in Baker's audience did not know he was referring to Bush. Baker does not like slashing attacks, and seems not to know how to make them even when he tries.

"When he aims at the jugular, he hits the knees," one political observer here says.

Baker's style, as the senator acknowledged at a Beverly Hills fund-raiser last month, tends to reflect the intricate and usually civilized discourse of the U.S. Senate.

"They say 'distinguished gentleman' when they address each other in the Senate," observes Michaels, who directed Gerald R. Ford's 1976 campaign in New Hampshire. "That doesn't always work in presidential campaigns."

There are many theories for Baker's seeming failure to catch on. Some say he is too short or that his New South political style is too reminiscent of Carter's. Others say that his staff is weak, his organization poor and that he started too late.

But the biggest difficulty simply may be that Baker is, as he portrays himself, a general election candidate trapped in a primary election campaign.

"Baker would be a good candidate for president, and a good president, if we could get him there," says Peterson. "But in a primary election the middle-of-the-road candidate gets belted from both sides. That's what's happening to Howard Baker, and we started late. It's hard to come from way back."