Embattled Republican Party Chairman Bill Brock heads west for a showdown meeting with GOP presidential candidate Ronald Reagan today amid signs that the move to dump Brock is collapsing.

Sources within the Reagan camp both favorable and unfavorable to Brock expressed optimism that an agreement will be worked out with Reagan and campaign director William J. Casey that would lead to Brock's retention.

The agreement probably would bring in Drew Lewis, the former Reagan chairman in Pennsylvania, as Reagan operative on the Republican National Committee with a title yet to be determined.

Brock told the Washington Post earlier this week that he was "very willing" to work with Lewis. Persons close to the Republican chairman said yesterday that Brock wants to be cooperative and continue in his post if at all possible.

Reagan, who apparently became involved in the discussion yesterday for the first time, has been under mounting pressures from GOP leaders, including moderate governors and a wide spectrum of congressional leaders, to keep pressure, a conservative campaign to force Brock's removal has been steadily weakening. One Reagan source said that it is now extremely unlikely that Jerry Carmen, a hard-driving conservative who served as Reagan's New Hampshire chairman, would be brought into the campaign.

Lewis, while a Reaganite, is not considered to be a conservative ideologue.

He served as Gerald R. Ford's Pennsylvania chairman in a successful effort to stave off the Reagan forces in the 1976 campaign.

Reagan was described as anxious to avoid a confrontation within his own campaign at this time.

Several Republican leaders of various persuasions have expressed concern that the Reagan campaign is being rent by ideological infighting at the time it should be consolidating for the fall campaign.

The infighting, say these GOP officials, is undercutting the anticipated impact of a series of 'unity dinners" Reagan is giving to help the candidates he defeated pay off their campaign debts.

Brock is scheduled to participate tonight in Los Angeles at the second of these dinners. But the question of whether unity exists between the party's chairman and its prospective presidential nominee will be determined beforehand in a close-door meeting among Reagan, Brock and Casey.

One pressure point in Brock's behalf has been former president Ford, who praised Reagan and predicted his election when the two men met at Ford's offices in Rancho Mirage last week.

It was learned, however, that Ford strongly urged Brock retention in the private meeting that preceded the joint Reagan-Ford news conference Reagan values Ford's support, and is considered unlikely to take an action that would directly offend the former president so soon after this meeting.

The Reagan camp has been giving signals for several weeks that Brock would be retained as chairman. After the former California governor clinched the nomination, however, a movement of conservatives led by Sen. Paul D. Laxalt of Nevada, Reagan communications aide Lyn Nofziger and Carmen pushed for changes at the Republican National Committee.

These conservatives portrayed Brock as being insufficiently sympathetic to Reagan and objected to the hiring of several deposed Reagan staffers at the national committee. Brock has consistently pledged his own loyalty to Reagan and promised that the committee would devote its full energies to his election.

But at least until yesterday the fight raged back and forth within the Reagan camp and the committee because Reagan had not taken action of any kind.

Now, the candidate is described as realizing that the continued feuding is hurting his campaign. Within the Reagan camp, the betting is that it will be resolved today with the likelihood that Brock will stay.

Another voice of support, for Brock came yesterday from Sen. Howard H. Baker Jr. of Tennessee, a former GOP presidential candidate who is scheduled to attend the unity dinner tonight.

"I think it would not be a good thing to change horses in midstream," Baker said.