The Rev. T. F. Tenney had announced that "the featured ones here tonight are Jesus Christ and then Russell Long," and thousands of Louisiana Pentecostals in the tabernacle had murmured their approval. Now Sen. Russell B. Long - "Huey's boy," as he had been introduced - rose to the occasion:

"Many of you knew my father," Long began, a reference to the Kingfish, the demagogue and demigod assassinated in 1935 but still revered here. "I've tried to follow along in his tradition," said Long, who has been dubbed the Princefish in tribute to his political lineage.

"But I regret to say there's been a lot of mistakes made" in Washington, Long went on. "Some of those decisions of that Supreme Court have been very misguided decision. It leads me to wonder whether those justices start their sessions the way we do in the United States Senate - with a prayer. . . ."

At that, the tabernacle at the Pentecostal camp meeting grounds - big enough to hold four or five basketball courts and filled with worshipers - reverberated with amens and applause. As he usually does down home, Long had found his mark, this time with a huge audience.

"You Pentecostals do something that would be good for every church in the country," Long continued. "You don't leave anybody in doubt how you feel about a matter."

The crowd - which had been lifting the tabernacle roof with hymns, praying in tongues and otherwise expressing its fervor all evening - expressed its pleasure again.

"I want to make a little contribution," Long added, "so there won't be any doubt how I feel about the matter - the fine job you're doing here. It may not be much, but it's the largest contribution I've made at any one time."

He then handed the bespectacled and round-cheeked Rev. Tenney a check, and the reverend asked the crowd to applaud. Then Rev. Tenney said:

"I have a check here signed by Russell B. Long on the American Bank in Baton Rouge for five thousand dollars." The tabernacle was filled with high-pitched groans of appreciation and more applause. "Praise the Lord!" Rev. Tenney said. Then, after a theatrical pause he turned to Sen. Long and asked, "When are you running again?" Rev. Tenney then led the crowd in laughter.

As a matter of fact, Huey's boy Russell is running for reelection to the Senate next year for the sixth time. Long, 60, has no announced opponent and no apparent political problems, but he was still determined to scamper around Louisiana this week like a nervous rabbit.

In Washington, Russell Long is a shrewd, wily chairman of the Senate Finance Committee. He is one of the few old whales left in the Senate, a man who commands universal respect - if not always admiration - as a legislator and guardian of his favorite causes, particularly oil and gas. By Washington standards he is something of an elder statesman.

Down home he is still "Huey's boy," as he has been introduced more than once this week. Never a pretentious figure, here Long is just folks, less worried about his dignity than he is about having enough hard candy to throw to the crowd at a Fourth of July parade.

Long travels the state in Henry Heltz's white Mercury, crossing the flat countryside on ribbons of concrete that in many cases were first laid 50 years ago by his father.

Henry Heltz was a state policeman for 21 years, and now looks after Sen. Long's interests around Louisiana. Henry worked for Uncle Earl Long when he was governor. Henry used to contribute 5 percent of his salary to Uncle Earl at election time - the old "dee-duct" system of campaign financing, first perfected by the Kingfish.

Long calls out the names of most towns just before the first sign appears on the side of the road to announce them. Then he points to the street corner or courthouse steps where he made a speech in one of his earlier campaigns, or perhaps in one of Uncle Earl's.

Wednesday morning in bogalusa, it turned out that there wasn't enough candy in the trunk of Heltz's car to throw in the parade. Nothing is worse than an insufficient supply of candy, Long explained - better to throw nothing than to disappoint part of a crowd by running out.

The candy supply in the trunk was more than adequate at the next stop - the Fourth of July parade in Elton, a dilopidated one-story town in central Louisiana. There, Russell Long energetically tossed hard candies from his car to the spectators along the parade route, competing in this endeavor with a variety of other candidates for governor, sheriff, tax assessor and more.

These visits are the first phase of his reelection campaign. Long says he knows he cannot count on old-fashioned courthouse campaigning to win in 1980, however.

"My uncle Earl won the last old-time campaign without television in louisiana," he said yesterday, racing through the pines of the Louisiana countryside in the white Mercury. That campaign was in 1952.

Uncle Earl didn't like the television age and its emphasis on imagery. He ridiculed his opponent in 1952 for wearing pancake makeup and " $400 suits."

"You know what one of those $400 suits would look like on Uncle Earl?" Russell Long quoted his uncle as saying again and again in 1952. "Like socks on a rooster."

Sen. Long has made his peace with television. As chairman ot the Senate finance committee and a personal friend of many of the country's captains of industry, he can count on a huge campaign war chest to finance television advertising next year even without the "dee-duct" financing system.

In each of his previous five reelection campaigns, Long has won by at least a 2 - to - 1 ratio. But this year he is concerned. Long admits that his approval rating in polls has slipped a little since he last ran in 1974, and now is just below 60 percent.

So this time he is working harder. When he last went to the annual Pentecostal camp meeting, just before the 1974 election, Long donated $2,500 to the church.

As he did at the camp meeting, Long looks for subjects and themes that he thinks will appeal to each audience he meets. With local officials he talks about revenue sharing.

"I think you ought to have a cost of living allowance on your revenue sharing," Long told a group of local officials and businessmen in Alexandria this morning.

At most stops, Long has promised that this Congress will vote for catastrophic medical insurance "without putting any new taxes on you."

The senator's early campaign patter includes some grim news for President Carter; Long is speaking critically of the strategic arms limitation treaty (SALT II). "I guess I'd be leaning against it," he said today. "To a lot of us it's looking more and more like it's not a very good agreement."

Without the southern moderates and conservatives whom Long could help influence to vote for the treaty, its chances in the Senate are poor.

Last year Long voted for the Panama Canal treaties and he says this has proven to be a continuing political liability. "You ought to have at least one of those in 30 years," Long said, decribing his Panama vote as one he knew would be troublesome, but also thought was cbest for the country."

But now it's campaign time again and that brings out old political instincts in Huey's boy Russel. "Survival," he observed today, "is the first law of nature." CAPTION: Picture, SEN. RUSSELL LONG . . . rating in the polls has slipped