President Reagan, trying to prevent a lonely landslide, sought today to rescue Republican candidates his strategists consider to be likely losers on Tuesday.

Reagan campaigned here for incumbent Republican Sen. Roger W. Jepsen and in Arkansas for Ed Bethune, the GOP Senate candidate.

Both are trailing by wide margins in various polls, but a Reagan strategist said the president wanted to demonstrate that he was doing everything he could to help other Republican candidates. The White House view is that Reagan's strenuous efforts in the last week of his campaign will add to Republican strength in the House and foster post-election GOP unity.

Departing from his standard stump speech at a rally of more than 10,000 partisans at Little Rock, Reagan denied that he would propose tax increases in a second term, as Walter F. Mondale has charged.

"And any of those rumors . . . that are being floated around desperately in these last couple of days about some suspected tax increases from our side -- over my dead body, don't you believe it," Reagan said. Later, He joked that he hoped "no one took me literally" about his "dead body."

The Washington Post reported Friday that Treasury Secretary Donald T. Regan favored eliminating federal income tax deductions for state and local taxes as well as taxing some unemployment-insurance payments and workmen's compensation payments.

Asked about this story today in a session with reporters, the president said Regan has "already rejected those proposals." Reagan said he would not allow tax increases "under the guise of tax reform."

The president also commented on the CIA manual that advocated "neutralizing" officials of the Sandinista government. "There was nothing in that manual that talked assassination . . . , " he said, adding that use of the word "neutralize" was a bad interpretation of "remove -- meaning remove from office."

Reagan also said a purported Soviet implication that the United States was involved in the assassination of Indian Prime Minister Indira Gandhi "was probably the world's biggest cheap shot . . . . "

Reagan was happy today as he campaigned in Arkansas, Iowa and Wisconsin. Here in Winterset, he visited the boyhood home of the late John Wayne, whom Reagan called "a great patriot and friend."

The president was buoyed by polling reports that he held commanding leads in every state except Iowa and Mondale's homestate of Minnesota, two states in which the candidates are neck and neck.

Reagan bid for farmers' support and said he "needed" Jepsen back in Washington, where he had served "with honor and skill."

Reagan blamed Iowa's current agricultural hardships on the "ineffective and totally wrong-headed grain embargo" imposed four years ago by President Jimmy Carter in response to the Soviet invasion of Afghanistan.

Last week, Carter's secretary of agriculture, Bob Bergland, said the "impact of the grain embargo is a fairy tale," a remark Reagan today referred to scornfully.

"If they think that embargo was a fairy tale, then let them come to Iowa," Reagan said. "Let them go to Illinois and Missouri and Ohio to see how farm families have had to struggle to recover from that 'fairy tale.' The only thing that embargo did was to . . . empty your wallets."

In Little Rock, Reagan combatively defended his economic policies and said those who say his tax cuts unfairly benefited the rich are "lying in their teeth. We cut them across the board for everyone."

Reagan, who last supported a Democratic presidential candidate when Harry S Truman ran in 1948, continued to insist that he stands for traditional Democratic values.

"I changed parties when the leadership of the Democratic Party changed course," Reagan said in Little Rock. "Its current leaders have made that once-great party into the plaything of the left, the hobby of the elite and the home of the special pleaders. They don't represent America anymore . . . "