Nearly two years ago, Rep. John B. Breaux (D-La.) rejected Democratic Party pleas to oppose President Reagan's budget cuts, boasting that, while his vote was not for sale, "I can be rented."

At that time, administration support of sugar price supports was far more important to Breaux than loyalty to a weakened Democratic leadership.

Breaux's defection came home to roost on Jan. 3. Meeting behind closed doors, the Democratic Steering and Policy Committee rejected his bid for a seat on the House Budget Committee, dismissing him and his 10 years' House seniority in favor of eight of his juniors.

The disciplining of Breaux was just one sign of a major shift in political strength in the House, providing the structure for Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), Majority Leader James C. Wright Jr. (D-Tex.) and the rest of the leadership to try to present a Democratic legislative alternative to the Reagan program.

In organizing the 98th Congress, the House Democratic leadership used its new strength and confidence to bolster its power by a margin far exceeding the Democrats' 26-seat gain in the 1982 elections.

The revamping of the membership of key committees, particularly Budget, increased not only the Democrats' numerical strength but also the leverage of the liberal wing of the party. In addition, these new committee members are, almost without exception, Democrats with a history of loyalty to the leadership.

On the Budget Committee, which has been the vehicle for the development of economic policy, the ratio of Democrats to Republicans will increase from 18 to 12 to 20 to 11.

Rep. Phil Gramm (D-Tex.), the conservative "Boll Weevil" southern Democrat who last week resigned his seat to run as a Republican, temporarily lost his committee assignment. The eight new Democratic members of the committee include six liberals, and the other two, Reps. Martin Frost (D-Tex.) and Butler Derrick (D-S.C.), are moderate-to-conservative, but are firm supporters of the leadership.

This is a sharp change from two years ago, when O'Neill's and Wright's fear of losing even nominal control of the House was a major factor in their decisions on committee assignments.

They desperately attempted to appease the southern conservative wing of the party with posts on the most prestigious committees, only to see many of these Boll Weevils repeatedly defect to Reagan on critical tax and budget votes.

Two years ago, the need to appease the conservatives in the Democratic caucus boxed O'Neill into the position of having to oppose a liberal attempt to change the rules to restrict "riders" on appropriations bills. Conservatives often used such amendments to force limitations on government spending.

Last week, by contrast, O'Neill won restriction of such riders with the support of every Democrat, including conservative southerners, except for two, Gramm and Rep. Larry McDonald (D-Ga.).

While rejecting Breaux's bid for the Budget Committee, the leadership gave Rep. Buddy Roemer (D-La.) a seat on a middle-level committee, Banking, Finance and Urban Affairs, only to discover that Roemer had threatened publicly to bolt to the GOP if he did not get on the committee.

Roemer, warned that he would be challenged by the full caucus of Democrats, was forced to make what amounted to an apology to the caucus before it granted final approval.

Rep. Robert H. Michel (R-Ill.), the minority leader who for two years worked with the Boll Weevil Democrats to engineer bipartisan majorities for Reagan's economic program, added, "I just don't see that on critical issues, that we have any real hope of the same type of coalition."

The added Democratic strength, however, creates a number of potential liabilities, including:

Democrats will not have the luxury of making political gains based primarily on Republican economic failures. Instead, their increased strength will generate pressure to produce their own economic program, beginning with a budget, to be negotiated with the Repubican-controlled Senate.

Some Republicans contend that the Democratic leadership has made the Budget Committee too "muscle-bound," that its moderate-to-liberal wing is likely to produce a budget that is not only unacceptable to Republicans but also is unacceptable to enough conservative Democrats that it can't pass the House.

But, just as the Democrats moved to the left, the Republicans used their committee assignments to strengthen and reward their right wing, which suggests that the GOP also is not preparing for compromise.

For example, a third-term moderate Republican, Olympia J. Snowe of Maine, who has been a member of the GOP's moderate "Gypsy Moth" faction, was denied membership on three major committees, Appropriations, Budget, and Energy and Commerce. In each case, she was rejected by the Republican House leadership in favor of more junior representatives, including two freshmen.

While the Democratic leadership appears to have established firm control over the Budget Committee, threatening the authority of the chairman, James R. Jones (D-Okla.), who is to the right of most of his Democratic committee colleagues, it is by no means clear that it has similarly collared the Ways and Means Committee.

In 1981, the Ways and Means Democrats got into a bidding war with the Reagan administration for the favor of the oil industry, which resulted in the addition of multibillion-dollar tax breaks for the industry and other special interests.

In 1982, when public support for tax reform reemerged, the committee abandoned its role of initiating tax legislation and deferred completely to the Republicans who control the Senate Finance Committee.

This year, the partisan ratio of Ways and Means, which already heavily favored the Democrats, remains the same, 23 to 12. The Democrats added two relatively liberal members and one moderate southerner, Reps. Byron L. Dorgan (N.D.), Barbara B. Kennelly (Conn.) and Ronnie G. Flippo (Ala.), but the committee's aggressiveness, or lack of it, will continue to depend on the chairman, Dan Rostenkowski (D-Ill.).

In the case of the other major committee, Appropriations, Democrats increased their advantage over Republicans from 33 to 22 to 36 to 21, and filled four vacancies with liberals and moderates.