The FBI, Secret Service and Capitol Police are investigating a religious cult leader in Wheaton to see if he poses a threat to the lives of President Reagan and Sen. Mark Hatfield (R-Ore.).

The self-styled prophet, Fernando L. Dasbach, has also confronted area churches with a "Declaration of War" warning that clergy "by the tens of thousands will die" unless they convert to his views.

In a bizarre and rambling talk at a recent religious meeting, Dasbach, a follower of the 23-year-old Guru Maharaj Ji, warned that Hatfield "will die and Reagan will die; Reagan will die first."

Dasbach had made almost identical statements in letters to Hatfield's office several weeks ago after the senator refused Dasbach's demand to address a Senate prayer group, according to a spokesman for the senator. The group is restricted to members of the Senate.

After investigating Dasbach last month, the Secret Service reported back to the senator that Dasbach was "not a danger," that the remarks were made in a theological context and not intended as a specific threat to the lives of Hatfield or the president, according to Hatfield's office.

But authorities reopened the investigation when they learned of Dasbach's statement in a public meeting April 5 at St. Luke's United Methodist Church in Northwest Washington. "We thought we had the matter in hand," said a federal investigator. "He's not supposed to be making these kinds of threats."

Dasbach, a onetime social worker for the Council of Churches of Washington, was converted to the Guru Maharaj Ji's world about nine years ago, according to his German-born wife, Annmarie, and subsequently founded what he calls the Regenbogen Verlag -- German for rainbow publishing.

Although Montgomery County police and some cult experts believe that Dasbach and his followers are an offshoot of the Guru Maharaj Ji's Divine Light Mission, he and his associates deny that they are an organized group.

Little is known or available about the size or nature of the group, whose members have evaded questions about such matters. They operate under the banner of the Regenbogen Verlag, which has the same address and phone number as Dasbach's house on Holdridge Road in Wheaton, where federal agents said they encountered people in saffron robes "dancing around" when they investigated him last month.

Dasbach and his followers alarmed some area clergy and their congregations when some of them interrupted a number of church services on March 15 to present the "Declaration of War." The long, rambling document, with anti-Semitic and anti-clerical language, asserted that God "has declared war against the infidels of religion."

In apocalyptic terms it concluded: "All heads will roll, but first the heads of the priests. They are to be the first sacrifice. One by one they will perish. . . By the year 2001, not one Jew will be left. Only true believers of God shall remain."

The Rev. Earl C. Kettler, assistant pastor at the Lutheran Church of St. Andrew in Wheaton, who said a stranger handed him a copy of the declaration while the congregation was singing a hymn, expressed anxiety over the whole affair."I'm concerned," he said. "I feel anxious now knowing what is going on. Who is going to do the killing?"

Tom Getman, a Hatfield aide who is active in religious affairs, expressed concern over the war declaration's potential for encouraging unstable individuals to fulfill the prophesied deaths of clergy.

"The problem is not so much with Dasbach. The problem is with the people he gets all riled up," he said.

Getman quoted from one of what he termed Dasbach's "nasty letters" to Hatfield: ". . . I represent the godfather whom many call a cult leader. . . hEveryone should join. . . It is the only cult which will not kill its own members. It will only kill the opposition."

The war declaration was followed a few days later by engraved, formal invitations -- sent by registered mail to 128 area churches, Dasbach said -- "to Attend the Wedding Feast of the Father."

For the few who showed up at the gathering Sunday night at St. Luke's, which the church's pastor, the Rev. Orin W. Dooley, said he agreed to host as a matter of "Christian hospitality," the "wedding feast" turned out to be a series of harangues by Dasbach and his tan-and-white-clad followers, aimed at attracting new converts.

Dasbach refused to answer any questions after his hour-long attack on ministers, all Christians, Jews, Moslems, the press, pesticides and several members of the audience. He has since rebuffed all further attempts to ask him questions.

During his Sunday address, however, he said his message was "a letter [to priests and ministers] signed by the Creator, saying, 'If you do not obey . . . I'll kill you.'" He added that ministers "by the dozens, by the hundreds, by the thousands, by the tens of thousands will die," if they fail to acknowledge the divinity of his god.

"God punished the Jews who denied Christ," Dasbach said, and condemned them to "punishment for the last 2,000 years. He created Hitler. He can create more Jews to be stuck in the ovens."

Although he drew heavily on Biblical allusions to reinforce his arguments, Dasbach at one point snatched up a Bible from the pulpit, shook it angrily and, declaring "there's nothing holy about that book," heaved it the length of the room over the heads of his stunned listeners.

Some observers of the cult scene, including Montgomery County police, believe Dasbach's group split from the Divine Light Mission, which achieved some notice in the early 1970s by attracting such counter-culture heroes as Rennie Davis, one of the Chicago Seven.

Dasbach denied any current affiliation with the Divine Light Mission, though he repeatedly professed his devotion to its youthful leader, who is now known as Sant Ji Maharaj.

Efforts to learn about Dasbachhs ties with the Divine Light Mission brought a curious response from the cult's leaders in Miami Beach. Joe Antel, who for years has been the official press spokesman for the mission and the guru, failed to return repeated phone calls. But through an intermediary, a Miami Beach rabbi who heads an anti-cult group, Antel let it be known that Dasbach has been discredited and disavowed by the Divine Light Mission.