THE SENATE WILL vote on Tuesday on a motion to curtail debate on a constitutional amendment that provides for the election of the president by popular vote. The motion should be passed. Its purpose is to focus debate on the electoral college by getting rid of extraneous riders. That is as it should be: The issue is important enough to be taken up seriously and alone.

The popular vote amendment has been sitting around Capitol Hill for more than 10 years. One version passed the House in 1970 but died in a Senate filibuster. The current version is now threatened by an effort on the part of its opponents to add to it their own pet constitutional amendments, like the one to balance the budget. The idea is that such excess baggage will drag down and defeat this effort to abolish the electoral college.

In one sense, this amendment embodies the most radical change ever proposed to the Constitution. It would eliminate state boundaries as a factor in the election of the president and give the choice directly to the voters for the first time. Under its terms, the presidential candidate who gets the most votes will be elected, regardless of where those votes come from. If no candidate gets 40 percent of the total, a runoff would be held between the top two candidates. This system was recommended by the Senate Judiciary Committee after 42 days of hearings over several years during which many alternative proposals were considered.

There are those who regard this as a formula for political chaos. Charges of fraud and demands for recounts could delay the decision on who had won a close national contest, they say. Or, many serious candidates might make the race every four years, instead of two or, occasionally, three. Candidates would ignore the small states and focus on population centers. Voters would lose an important sense of citizenship in a state.

Ranged against this are two strong arguments. The first is that a vote cast for president ought to count for the same value whether it is cast in North Dakota or New York; but it does not when small states have proportionally more electoral votes than large stats. The other is that until the electoral college is abolished, the country will run the risk every four years of electing a president who has finished second in the popular vote. These are the best reasons that the Senate should vote cloture on Tuesday and approve the direct-election amendment itself later in the week.