Bob Michel, a jovial conservative from Peoria, Ill., was elected leader of the House Republicans yesterday, defeating combative Guy Vander Jagt of Michigan in a 103-to-87 secret ballot.

Exhilarated by their increased numbers, the Republicans seemed fairly lusting for Democratic blood in their first organizational meeting, as Michel declared he was ready to begin "commando raids" if the Democrats don't agree to reduce their "exorbitant" majorities on key committees.

House Democrats renamed by acclamation Speaker Thomas P. (Tip) O'Neill Jr. (Mass.) and Majority Leader Jim Wright (Tex.), who then appointed Rep. Thomas Foley (Wash.) to the third-ranking leadership post of majority whip. Louisiana moderate Gillis Long was elected caucus chairman by a vote of 146 to 53 over Charles Rose (N.C.)

Conservative Democrats who had served notice on O'Neill that, as the swing votes in the House, they want a bigger say in party leadership did noticeably better than usual. The leadership's first four nominations to the important Steering and Policy Committee consisted of three Southern conservatives -- Tom Bevill (Ala.), Bo Ginn (Ga.) and Wes Watkins (Okla.) -- and O'Neill's Boston buddy, Joe Moakley (Mass.). Rep. Bill Alexander (Ark.), a conservative, was appointed chief deputy whip, replacing Dan Rostenkowski (Ill.), who will chair the Ways and Means Committee.

The Republicans elected an outgoing Mississippian, Trent Lott, to the No. 2 spot of whip in a 96-to-90 vote over Bud Shuster of Pennsylvania. In the contest for chairman of the House Republican Conference, Jack Kemp of New York, a close associate of Ronald Reagan and outspoken tax cut advocate, won over John Rousselot of California.

The only woman running for a contested leadership position in the House, Majorie Holt of Maryland, was defeated 68 to 99 for chairman of the Republican Policy Committee by Richard Cheney (Wyo.), President Ford's former chief of staff.

In a rousing speech after his victory, Michel, who had to fight his image as a nice guy accustomed to playing second fiddle, declared that if the Democrats' strategy was to "dig in and fight. . . Bob Michel never ran away from a fight whether it was in the foxholes of World War II or in my most recent mugging."

Michel, 57, was brutally beaten and robbed outside his Capitol Hill home in August 1978.

A 24-year House veteran, Michel has been minority whip for the past five years, a thankless job during a time when Democrats outnumbered Republicans two to one. Now that the GOP is catching up -- the new makeup will be 192 to 243 -- many of Michel's colleagues simply felt he deserved a promotion.

Vander Jagt, who won acclaim as an aggressive orator and a tireless campaigner for Republican candidates, won a majority of votes among the 52 GOP freshmen, many of whom attributed their victories to him.

It was a contrast of styles between two equally conservative midwesterners. Vander Jagt was more the flashy, feisty bulldog while Michel was portrayed as basically a compromising legislative technician.

"I don't personally crave the spotlight of public opinion," Michel told his colleagues. "My job is to orchestrate your many talents. I know some of you prefer to speak quietly, like woodwinds, and some very loudly, as brass and percussion. But our measure of success is how well we harmonize."

Michel sought to dispel rumors that he would retaliate against Vander Jagt supporters in handing out committee assignments. "Bob Michel keeps no little black book in his desk drawer," he said. "Mine is a large green one with a lot of names and plans for the future that will include each and every one of you."

Since the Senate and White House have gone Republican, the House is the last Democratic stronghold and thus will be the focus of attention, Michel said. "So we've got to be on the offensive. our bottom line is the enactment of the Reagan program."

The Democratic caucus accepted the leadership's proposal to add chairmen of four key committees -- Rules, Ways and Means, Appropriations and Budget -- to the Steering and Policy Committee, which originates party policy. This will raise the committee's membership to 12 elected by regional caucuses and 17 members appointed by the party leadership of the House.

Others named to the policy committee yesterday were Reps. Geraldine Ferraro (N.Y.) in her capacity as secretary of the caucus; W. J. Tauzin (La.), elected by freshmen Democrats, and three other O'Neill appointments -- Charles Rangel (N.Y.), Norman Mineta (Calif.) and Patricia Schroeder (Colo.).

Today, House Democrats will vote their first response to the November elections when they choose a chairman to lead the House Budget Committee for the next two years. The choices are David Obey (Wis.), a liberal; James Jones (Okla.), a pragmatic conservative, and Paul Simon (Ill.), who fits somewhere in between. All the other committee chairmen will be elected by the caucus in January.

As whip, Foley will resign as chairman of the House Agriculture Committee; next in line is Rep. E. (Kika) de la Garza (Tex.).