It's troublesome enough when the president and Congress get into arm wrestling over foreign policy power- sharing to the point that it is impossible to know where the United States stands on big issues. But it's even more revealing of the rancid state of relations between the two branches when they can't even do the routine things right -- like sending an American ambassador to the People's Republic of China.

The Reagan administration has been struggling to change its ambassadorial guard in Peking for at least four months and the end is not in sight. In July, after meticulous soundings, President Reagan sent to the Senate the name of Winston Lord, a Kissinger whiz kid in the Nixon days with an impeccable Republican pedigree and the full backing of the State Department. The usual confirmation hearings were conducted by the Senate Foreign Relations Committee that same month.

But not until September could the committee get around to a vote. Approval was overwhelming. The voting pattern was familiar. It was 16-to- Helms.

That would be Jesse Helms, of course, the nay-saying North Carolina Republican whose good-ole-boy manner belies the Senate's meanest streak. Helms voted by proxy, not even bothering to show up and not having raised any credible questions about Lord's credentials.

The merits of matters are not Helms' test. Just as he made a shambles of the Reagan administration's efforts to put together a foreign policy team in the early months of the first term, so Helms is making a mockery of the Senate confirmation process once again. He is invoking "senatorial courtesy," an unwritten club rule allowing one member to stall a final confirmation vote on the Senate floor. Usually his pretext has to do with some fancied guilt by association with one or another "liberal" line of thought that doesn't fit Helms' way- out right ideology. This time around the Helms hangup has to do with official policy on right to life -- Peking's policy, that is, and what the Reagan administration is doing about it.

At the bottom of all this lies an arguable issue and a monumental problem. With one-fourth of the world's people, the Chinese have reason to worry about the implications of unchecked population growth. So they are practicing draconian government intervention: one child per family -- by decree. Even the Chinese press has reported "terrible abuses," according to knowledgeable China watchers.

The United States neither endorses nor supports this Chinese effort directly. But it does contribute to a United Nations population control program, which in turn contributes to Chinese family planning. Earlier this year, the administration registered its protest against the excesses in Chinese population control. The U.S. aid agency withheld $10 million of the $46 million annual U.S. contribution to the U.N. program (which does not itself condone "coercive" family planning policies). The $10 million roughly corresponds to the U.S. contribution to China's population control. But the withholding of the money, it's conceded, is "symbolic" so long as the United Nations continues to offer any aid to China's population control.

So Helms and a handful of like- minded Senate conservatives would use the Lord appointment to knock the administration, the U.N. and the Chinese government upside the head -- to put an end to any U.S. financial support for any U.N. population control program without an assurance that American money will not be used to support enforced abortion or involuntary sterilization. Helms & Co. have told the president that in exchange for a guarantee the administration would be hard put to give (without closing down all aid to the United Nations or at least to its population control program), Lord's appointment will be set free.

So goes the day-to-day business of the world's greatest deliberative body: taking innocent hostages. Talks are under way to end the impasse. Senate Majority Leader Robert Dole has said that "if there is no agreement, I'm duty-bound to call up the nomination and have a vote on it." That he is.

The republic will not fall for lack of an American ambassador in Peking, even though in recent weeks, what with the visit of Vice President George Bush and a meeting of the government's central committee, it would have been useful to have the new ambassador on the job. But the republic does look all the more ill-constituted to handle really big deals when it can't even do little deals right.