Democrats and Republicans in the US Congress are probably the most undisciplined legislators in the world when it comes to following a party line. While lawmakers in other democracies vote lockstep with their parties on major issues, the Democrats and even the more-unified GOP on Capitol Hill must plead and cajole to bring members in line. Often those members care more about the views back in their home districts than party loyalty, and there is little a party can do to change them.
One of the most glaring examples is Rep. Charles E. (Buddy) Roemer III, a young Louisiana Democrat who arrived as a brazen freshman in 1981, threatening not to vote for Rep. Thomas P. O'Neill Jr., his party leader, to be reelected speaker of the House.
Such an idea is political heresy because the vote for speaker determines which party controls the House. But taking on the liberal Massachusetts Democrat pleased Representative Roemer's conservative Louisiana district.
As it turned out, Roemer softened his stand after he met with the Speaker and shared jokes about Boston and Harvard University (where Roemer studied), while disagreeing on many political issues. On opening day, he voted for O'Neill and then committed the second worst party crime by voting against the Democratic rules for organizing the new Congress.
It was a shaky start for the newcomer. But four years later his daring has won approval at home and hardly hurt him in Washington.
''I did feel my independence cost me some standing here,'' says Roemer. The party denied him his first choice of committee assignments that first year, but he quickly began rebuilding the bridges he had burned. By the next Congress he had won a slot on one of the most sought-after committees, Banking, despite grumbling from some fellow Democrats. He prevailed, he says, by hard work and meeting with all of the Democratic leaders.
''They didn't ask me for a single promise,'' he recalls. ''All they asked me was, when possible and where possible, to listen to them and work with them.''
So Roemer developed his own rule of party loyalty. ''As soon as I know I'm approaching a limit'' and cannot vote with the Democrats, he says, ''I let the leadership know.''
The Louisiana congressman must be very busy communicating with House leaders, since he remains one of the most unreliable Democrats of either house. Last year he opposed his party in 73 percent of the party-line votes, according to a study by Congressional Quarterly.
Even the far better-disciplined Republicans have straying members. Rep. Olympia Snowe, as deputy whip for the GOP, has the task of trying to improve party unity. But she confesses of her own voting record, ''I'm not a consistent supporter of the party line.'' She voted her Maine constituents' views and against the GOP in 53 percent of party-line votes last year, according to Congressional Quarterly.
''Most Republicans are in tune with the administration,'' so party loyalty is easy for them, she says. But about 20 members, dubbed Gypsy Moths, from the Northeast and Midwest, have districts with liberal leanings that sometimes push them toward the Democrats.
Both congressional parties are ''less rigorous than any state legislature,'' says Rep. Thomas S. Foley of Washington State, the Democratic whip in the House. ''We don't expect them to vote our position except on votes for the speaker and the rules,'' he says, and clearly he cannot always count on them for those.
The term party ''whip,'' which has been traced to an English fox hunting term for the ''whipper'' who kept the hounds together, is hardly an accurate term, according to Representative Foley.
''There aren't very many, if any, sticks,'' he says. ''There're a few carrots'' in the form of future assignments to powerful committees or special commissions. But while the leadership could once insist that members follow their committee chairmen, ''arm twisting almost guarantees failure'' in the modern Congress, according to Foley.
The same is true in the upper chamber, according to Sen. Ted Stevens (R) of Alaska, assistant majority leader and Senate whip.
''LBJ used sanctions,'' Senator Stevens says of one-time Senate majority leader Lyndon B. Johnson.
''Things were different then,'' he says. ''When I first came here, the leadership had control of the stationery. They'd leave the impression you wouldn't get it if you wouldn't (cooperate).''
Party discipline on Capitol Hill ''is never going to be like it was,'' says Christopher J. Matthews, an aide to House Speaker O'Neill. ''It used be that members dearly needed committee assignments'' to get reelected, he says, so they cooperated with their leaders. Now members go directly to voters via television.
While party organizations once picked candidates and controlled them once elected, politicians today no longer rely on party bosses. Democrats, especially , get very little campaign funding from their party and must raise their money by individual effort or from political-action committees tied to labor or business.
Republicans, with their successful direct-mail fund raising, have millions more dollars to offer, as well as training schools for candidates and their campaign advisers. Although Republicans in Congress steadfastly deny that the campaign chest gives the party clout over members, the GOP nevertheless is far more unified than the more diverse Democrats.
''The only sword we've got is talk,'' says Stevens.
But for Republicans, that talk can be powerful because it often comes from the President or from members of his Cabinet. Even top party contributors have been known to get into the act, telephoning members.
GOP unity hit a high in 1981, in the wake of the Ronald Reagan landslide that helped the party capture the Senate, win 33 seats in the House, and convince some Democrats to join the conservative bandwagon. Senate Republicans posted an estounding 81 percent record of voting the party line. Republican representatives united with enough Democrats to pass the key Reagan budget and tax-cutting programs in the nominally Democratic-controlled House.
Senate minority leader Robert C. Byrd (D) of West Virginia, bitterly flayed the ''Reagan robot'' Republican voting at the close of the first Reagan year.
Republicans view it differently.
''I think the Republicans were trying to give the President a chance,'' says an aide to Senate majority leader Howard H. Baker Jr. (R) of Tennessee. ''We find it pleasing to say you had 52 Republican senators standing together 44 times on budget votes.''
The unity has now somewhat disintegrated in the Senate. Many moderate Republicans almost lost their seats in the 1982 elections after having voting for the Reagan budget and tax cuts. ''Last year you had some people who saw close friends survive close races,'' says the Baker aide. ''This year everybody's trying to stake out their own ground.''
The last elections, which also returned 26 Democratic House seats, sent a similar message to House Republicans. They are now more cautious in backing the President, although Representative Snowe points to a growing Republican ''solidarity'' that ''reminds me of 1981.''
As the fall elections draw nearer, both parties are beginning to show more partisan colors. Democrats are trying to carve out their clear differences with the Reagan administration on foreign policy, deficits, and ''fairness,'' while Republicans will be trying to earn points by demanding action on a balanced-budget amendment to the Constitution and crime legislation. Among the chief partisan issues:
* Foreign policy, long considered a bipartisan area, has moved into the political arena because Democrats see it as a Reagan vulnerability. Thus, the question of United States aid for a covert war against the leftist government in Nicaragua becomes a partisan issue, with the Democrats in the House opposing it and the GOP Senate supporting the assistance.
Partisanship was even more apparent last year in the Senate when all of the Democrats voted against the President's policy of keeping troops in Lebanon.
* Weaponry, which has usually enjoyed bipartisan support, has turned into a partisan issue in the House, where the Democratic leadership led the fight against the MX missile.
* Federal budget deficits continue to be a major issue dividing the two parties, with both sides trying to find a favorable position for the elections. Democrats are insisting on higher taxes and less defense spending, while Republicans are seeking fewer new taxes and domestic spending caps.
But in these issues, members of Congress who are hoping to be reelected next fall will be focusing more on constituents at home than party leaders in Washington.