The bronc rider was thrown off his horse, the smoke jumper didn't jump and the judge found himself bedeviled with questions about the Panama Canal. It was just another day's episode in the strange U.S. Senate primary campaign in Montana.

The bronc rider, for all of 3 1/2 seconds, was Rep. Max Baucus, a cheerful, 36-year-old two-term House Democreat who aspires to fill the Senate shoes of the late Lee Metcalf.

On Saturday, at the annual Buckin' Horse sale in this eastern Montana cowboy town, Baucus also was trying to emulate Edward M. Kennedy, who successfully rode a bareback bronc here in 1960 as a campaign stunt in his brother John's presidential race.

It was a tough day for Baucus, who started by finishing last in a donkey-pulling contest down the town's main street. Then he was sent sprawling into the mud of the Miles City Fair-grounds arena by bronc No. 777, a spirited animal that was sold to a rodeo dealer for $700 for its efforts in unseating Baucus.

For all of this, Baucus is considered a faraway front-runner in his effort to unseat the short-team Senate incumbent Paul Hatfield, a 50-year-old attorney who spent his 17 preceeding years as a judge.

With time running out before the June 6 primary, Hatfield has yet to learn how to make the transition from the nonpartisan world of the judiciary to the practical one of a Democratic political campaign. Intelligent and witty in private conversation, he is a dreary platform speaker who sometimes forgets to speak into the microphone.

And in his personal appearances around this vast and far-flung state, the nation's fourth-largest in size though it has a population no larger than the District of Columbia's, Hatfield has been peppered with questions about his support of the Panama Canal treaties, which are unpopular in Montana.

Hatfield, whose political courage is respected even by politicians who regard him as a sure loser, was isolated on the Panama Canal issue because the state's other senator, Democrat John Melcher, voted against the treaties.

Baucus has said he would have supported the treaties, but he is not identified with them to most Montanans.

"It's like Max held the gun, but Paul pulled the trigger," says Larry Williams, one of three Republican candidates for the Senate nomination.

Montana has an open primary in which there is no party registration. On election day, registered voters are given both Democratic and Republican ballots and instructed to vote the ballot of their choice and return the other.

Williams and Ron Marlenee, the state's lone Republican congressman, believe that a number of conservative GOP voters would have crossed over to support Hatfield against Baucus except for the Panama Canal votes. Whether this would have been enough to make any difference in the outcome is widely questioned by various Democratic politcians. They say Hatfield's real problem is resentment against unpopular Democratic Gov. Thomas L. Judge, who plucked Hatfield off the state Supreme Court when Metcalf died last January in an effort to head off the popular Baucus.

The third candidate in the Democratic race is the smoke jumper - state House Speaker John Driscoll. He is a combative, 31-year-old Irish-American politican who made his living last summer as a $4.96-an-hour smoke jumper who parachuted into fires in Alaska and California.

Driscoll didn't jump in Miles City as planned, however. After high winds blew two markers that were dropped from his plane into a river more than a half mile from the fairgrounds, the jump was called off.

The cancellation may have symbolized Driscoll's underfunded and uphill campaign.

Though Driscoll is well thought of in the legislature, where he is considered a progressive and effective politician, he is less well known than his opponents and has far less money.

Estimates from the candidates are that Baucus will spend between $250,000 and $300,000, Hatfield a little more than $100,000 and Driscoll about $30,000.

At times Driscoll's effort has seemed more o fathletic one upmanship than political competition Discovering that Baucus was entered in a seven-mile race in Missoula, Driscoll entered, too, and ran it in 54 minutes, two minutes faster than Baucus. But it is likely to be his only victory over the popular congressman.

All three Democratic candidates - in the tradition of Mike Mansfield and Metcalf before them - combine a liberal and internationalist outlook with a stance of fierce protectionism for Montana interests on such issues as restricting beef imports, raising wheat subsidies and reducing grazing fees.

Driscoll is the most outspoken. Speaking at Wolf Point in the heart of Montana's Indian country the other day, he challenged his mostly white audience by supporting Indian rights and warning the town that it may lose some of its Amtrak service in the interest of railroad economy.

"John has switched his campaign from attacking Max to attacking the audience," Hatfield quipped after the event.

Baucus, for his part, has taken his licks at President Carter, who started out with limited popularity in Montana and appears to have gone down from there.

"President Carter has shown us that on-the-job training has certain deficiencies," Baucus said the other day to an appreciative audience of ranchers in Billings.

He went on to call the Carter farm program "lousy" and to suggest that the president doesn't understand the problems of the West.

Neither of the other candidates spends any time defending Carter, except on foreign policy issues. When a reporter said to Hatfield that he was coming off in the campaign as a Carter supporter, the senator rattled off a list of domestic issues - the farm bill, grazing fees, gun control and beef imports among them - when he opposed the president.

With few discernible differences apparent on the issues, the election is likely to come down to who Montanans think will be the most effective on their behalf. This, at least, is Baucus' message, whenever he speaks. He has an edge in experience over both opponents and an age advantage over Hatfield in a state that likes to elect senators young and see them acquire seniority.

Baucus, a Standford-educated lawyer whose parents are sheep and cattle ranchers in the Prickly Pear Valley south of Helena, also has a wide edge over his opponents in campaign organization and television advertising. He raised $43,000 at a Helena fund-raiser that featured actor Henry Fonda and also has tapped fund-raising sources in Washington, Chicago and Beverly Hills that his opponents are unable to match.

In contrast to Baucus' highly efficient and youthful campaign organization, Hatfield has depended on a few aides and his wife, Dorothy, an aggressive attorney who lacks partisan political experience. She became his campaign manager when Ronald P. Richards, an experienced politican, resigned after differences with Hatfield Reportedly, Richards wanted to run an aggressive campaign against Baucus, which Hatfield, who is unfailngly courteous to his opponents, has refused.

On the Republican side, the nomination. . . he down to a contest between Williams, a youthful investment counselor who has waged an aggressive campaign that has high-lighted his opposition to the canal treaties, and William Osborne, an insurance agent and former high school coach, who is considered the establishment Republican candidate.

The GOP winner, whoever he is, will be a heavy underdog in November in a state that frequently votes Republican in presidential elections but invariably sends Democrats to the Senate.