President Reagan launched his campaign today to keep the Senate in Republican hands with a speech denouncing "big taxers and big spenders" while opposing any cuts in the military budget to reduce the federal deficit.

Reagan's speech for Senate candidate Christopher Bond (R), the first of a scheduled series of fund-raising appearances, advertised the president's differences with members of his party who have urged that defense budget cuts or tax increases be included in any deficit-reduction package.

Calling for an increase in military spending, Reagan said the nation is "still making up for the folly of a decade ago when our defensive strength was permitted to erode.

"We must never permit that to happen again," he continued. "Strength is the greatest guarantor of peace."

Reagan's remarks on budget issues and world trade also demonstrated why some Republican candidates are distancing themselves from key administration policies even while trying to raise money by capitalizing on Reagan's popularity.

On the budget, for example, Senate candidate Bond has been an outspoken critic of wasteful military spending and has called for budget cuts in both the domestic and defense areas.

And in a state where protectionist policies are popular, the 800 partisans at the $500-a-plate dinner sat silently as Reagan restated his commitment to free trade.

"Protectionist measures would only raise the prices of what all of us buy and would likely result in retaliatory trade barriers against our farm products overseas," Reagan said.

The president called protectionism "the number one threat to American agriculture" and said the answer to foreign competition is to increase exports, not reduce imports.

Sen. John C. Danforth (R-Mo.), a leading proponent of protectionist legislation, is traveling overseas. Danforth also was not present for Reagan's most recent previous appearance in Missouri last Sept. 2 to appeal for tax revision.

The president described the farm bill, which he signed reluctantly in late December, as "about the best thing we could get through the Congress." In fact, the president had recommended that economic assistance to farmers be cut at least $15 billion below the amount Congress approved, and signed the bill only after Republican congressional leaders argued that a veto would mean a political disaster for the GOP.

Farmers in this area are not happy with the farm bill anyway. A small but vocal crowd of about 150 farmers booed Reagan loudly when he arrived and waved signs blaming him for their economic plight.

In his speech today, as in his news conference the night before, Reagan vowed to veto any tax increase sent him by Congress and renewed his support for the Gramm-Rudman-Hollings deficit-reduction law.

"Gramm-Rudman-Hollings is not a threat," Reagan said. "It is a tremendous opportunity for us to finally get rid of unnecessary federal programs and dramatically reduce the level of deficit spending."

Bond, a two-term governor, is seeking the Senate seat being vacated by Democrat Thomas F. Eagleton, who has served 18 years. His Democratic opponent is Lt. Gov. Harriet Woods, elected in 1984 after losing to Danforth in the Senate election two years earlier.

The Missouri race is considered by political operatives of both parties to be potentially critical for control of the Senate. Early public polls show Bond and Woods running neck-and-neck, while a Democratic survey this week gave Woods a 57-to-43 percent lead among voters who have decided.

Reagan spoke at an ultra-modern hotel that is part of a $140 million restoration project for St. Louis Union Station. The project was launched by a $10 million Urban Development Action Grant (UDAG), one of the federal programs that would be eliminated in the budget the president has submitted to Congress this year.

Reagan stopped at the fund-raiser en route to a brief vacation at his ranch northwest of Santa Barbara. He is scheduled to return to Washington on Saturday after delivering his weekly radio speech from the ranch.