For better or worse, until politics do them part, Republicans from Ronald Reagan to the lowliest House candidate will join hands on the U.S. Capitol steps Monday in a campaign union that has no parallel in modern American politics.

Brushing aside the risks, the Republicans are going further this year than either party has gone in memory to mesh their presidential and congressional campaigns.

It is to be a graceful waltz, they hope, that not only reflects credit on the skills and coordination of the GOP but also points up the awkwardness of the toe-stomping two-step of the Democrats over the past four years.

"We have a degree of consensus today that I'm not sure we could have achieved in other years . . . and I'm quite sure the Democrats couldn't do what we're doing," said Rep. Thomas B. Evans Jr. (R-Del.), an early Reagan backer in Congress and a principal campaign matchmaker and choreographer.

The ceremony itself is largely a razzle-dazzle event for the television cameras, "a giant photo opportunity that will have an impact of about 12 hours," said one skeptical Capitol Hill Republican.

Flanked by running mate and exchallenger George Bush, the Republican congressional leadership and about 150 GOP aspirants for the House and Senate, Reagan will attempt to lay out a program that all can work jointly to enact -- probably longer on goals than on troublesome specifics.

Less-ballyhooed events will include skull sessions with party pros and a late afternoon meeting, after the standard bearers and the press have gone, for representatives of money-giving political action committees to shop around among the congressional candidates for prudent investments.

As with most such events, the rhetoric on the Capitol steps will probably far outstrip the reality of the situation, which still basically boils down to the law of survival. In other words, the common bonds, which have already been strained by sporadic balkiness from within the Reagan camp and by a little edginess about Reagan's campaign gaffes, will be sorely tested if either partner begins to show signs of becoming a serious liability.

As for Reagan's mishaps and shrinking lead in the polls, he is still well ahead in key congressional target districts, according to Steven F. Stockmeyer, executive director of the National Republican Congressional Committee. "There may be some scratching of heads and wondering if this was such a good idea," said Stockmeyer, "but we haven't had much of that kind of talk so far."

On the whole, however, the Republicans claim that the "team" theme will be helpful in tapping voter frustration over government ineffectiveness, political squabbling and tensions between the White House and Congress. Potentially, the mutual benefits are so great -- both for the election and for governing afterward if the GOP is successful at the polls -- that it is worth trying at least, they say.

"We have an opportunity here that we haven't had in maybe 25 to 30 years," said Stockmeyer. "People are frustrated with government, frustrated with a lack of progress. Our opportunity is to say that the way to change things is to change the coach and change the team too."

For after the election, the opportunity is to "avoid another Jimmy Carter situation, with all the alienation between the White House and Congress," said Sen. Paul Laxalt (R-Nev.), Reagan's campaign chairman and, along with Evans, a key link in the coordinated drive.

The Republicans can already claim some benefits from their togetherness campaign -- none as splashy as their almost certain appearance on Monday evening's television news program but important in different ways.

Reagan, Bush, their families and campaign surrogates are being scheduled with some view toward critical House and Senate races, according to congressional sources.

Knowledgeable members of Congress are now, at the behest of congressional Republicans, being assigned to Reagan's plane in an effort to get more coherence on issues and fewer of the speech gaffes that plagued Reagan's campaign over the last few weeks.

And advance texts of important policy speeches, like Reagan's economic address to the International Business Council in Chicago last Tuesday, along with the logistical arrangements for potentially sensitive events like an upcoming meeting of Italian Americans, are now being circulated on Capitol Hill for embellishments or deletions.

The unity efforts may also be enhanced by the fact that Republican congressional candidates, far from being the poor relations of the GOP family, are well financed through the party apparatus this year -- receiving roughly $3 to $4 for every $1 that the Democratic candidates get from their party committees.

Stockmeyer estimates that 60 to 70 Republican House challengers will receive the legal limit of $35,000 from the national and congressional party committees, about half again as many as got the maximum in the past.

By contrast, Democratic candidates are left more to their own devices, which are often ample, but their debt to the party is diminished accordingly.

According to the Republicans, the idea of a coordinated campaign evolved largely out of circumstances, ranging from a fading of their old schisms and the emergence of figures such as Laxalt, Evans and GOP chairman Bill Brock, a former member of both houses of Congress -- all of whom had their feet planted in congressional and national politics.

Moreover, polls have shown a steady rise in the respectability of the Republican label as such, making a unified campaign less of a gamble.

But the catalyst, many Republicans said, was Carter's difficult relations with the Democratic Congress, which the Republicans could exploit by stressing the potential of a unified GOP presidential-congressional team.

Outnumbered 274 to 159 in the House and 59 to 41 in the Senate, they have only a modest chance of gaining control of the Senate and virtually no chance of winning the House this year. But they are gunning for enough gains in both houses to mount a real campaign for control in 1982, which would be midway through a Reagan administration if there is one.