The Senate's 53 Republicans will meet today to choose a successor to retiring Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) in an atmosphere of tension and suspense heightened by feverish, down-to-the-wire campaigning by the five contenders.

Barring a last-minute dropout, the field was described late yesterday as fairly evenly divided among the five: Sens. Robert J. Dole (Kan.), Pete V. Domenici (N.M.), Richard G. Lugar (Ind.), James A. McClure (Idaho) and Ted Stevens (Alaska).

The selection will be by secret ballot behind the closed doors of the tiny, museum-like Old Senate Chamber. Voting by the GOP senators to serve in the 99th Congress will continue until someone gets a majority, at least 27 votes. After each ballot, the lowest vote-getter will be dropped, gradually narrowing the field to as few as two.

With each successive vote expected to have a dynamic of its own, none of the contenders was predicting victory yesterday and some, such as Lugar, were expressing anxiety about making it through the first and second ballots.

Adding to the uncertainty, at least a dozen senators had not committed themselves as of yesterday.

One was Sen. Paul Laxalt (Nev.), President Reagan's closest friend in the Senate. Laxalt told the candidates he will keep his vote secret, lest it be interpreted as indicating a preference by the White House, which has stayed officially neutral.

It was apparent from candidates' nose counts that some senators have indicated support for more than one contender. On Monday, one name reportedly was on three candidates' lists of supporters.

The "Gang of Six" moderate-to-liberal Republicans caucused yesterday but, despite their hopes of swinging weight as a bloc, reportedly came to no decisions. They plan to meet again early today.

As chairman of the powerful Finance Committee and the most prominent national figure in the group, Dole remained the man to beat. But many senators have said privately that each of the others probably has as good a chance as Dole, whose presidential ambitions are a drawback in the Senate.

Domenici is especially well-liked but has drawn criticism as well as praise as chairman of the Budget Committee, both for prodding the White House to support deficit reductions and for acceding to its refusal to do so. Several senators said Domenici, once seen as an also-ran, was gaining strength, although all camps were claiming a surge in their direction.

Lugar helped many Republican senators as chairman of their campaign committee but got into the race late and had trouble getting started.

Lugar's problems are compounded by the fact that his election would eliminate him from the lineup for chairman of the Foreign Relations Committee, opening the way for a possible struggle between conservative Jesse Helms (N.C.) and the more liberal Charles McC. Mathias Jr. (Md.). It is a showdown many senators would like to avoid.

Meanwhile, yesterday, Conservative Caucus Chairman Howard Phillips said a poll of 500 North Carolinians who said they had voted for Helms showed 60 percent thought Helms should switch from the chairmanship of the Agriculture Committe to that of the Foreign Relations Committee if he could head a tobacco subcommittee of the Agriculture Committee.

McClure is the most conservative contender, a fact that gives him a solid bloc of votes going into the balloting but could limit his potential for expanding it in future rounds.

Stevens has an almost legendary temper that has caused problems for his colleagues in the past, but he is personally popular among many senators, including Democrats.

With personal factors expected to transcend issues and ideology, the five contenders' campaigning yesterday was highly personal, calling or visiting colleagues and attending senatorial social functions.

Baker was asked yesterday who his successor will be. He shrugged, laughed and said, "I have no idea."