"Politicians," according to Sen. John Danforth (R-Mo.), "have a tendency to say 'yes' to all men and 'no' to none."

This trouble has been noted before. Walter Lippmann described a politician as one who "decides not whether a proposition is good but whether it's popular; not whether it will work well and prove itself but whether the active-talking constituents will like it immediately."

And Sen. John F. Kennedy declared himself convinced that the desire to be re-elected exercises a strong brake upon "independent courage."

The problem of whether a politician should serve his conscience or his constituency has been with us at least since Edmund Burke. Sen. Danforth, who has made a certain mark during his first term for being refreshingly outspoken, thinks he has at least a partial solution.

Danforth has introduced a resolution proposing a constitutional amendment that would limit the terms of service in the House and Senate to 12 years, the equivalent of six terms for a senator.

His point is that without the hope of holding on to a lifetime job, senators and representatives, might vote their convictions. In fact, some of them might develop convictions, so that they would come to Washington, do what they think they ought to do and go home again - for good.

There is something to be said for this argument, and there are other reasons why Danforth's suggestion is worth thinking about.

For example, a greater turnover in Washington might renew the nation's interest in the electoral process. According to the polls, the country is in great doubts as to whether government can accomplish anything worthwhile - or, indeed, anything at all. Most people who are eligible to vote don't.

For another, the Danforth proposal would put an end to the system of power by longevity.

Sen. Russell Long (D-La.), for instance, has served in the Senate since 1948. At 60 and with at least two terms to go, he is likely to exercise more influence on the lives of his countrymen than any President they may elect. As chairman of the Senate's Finance Committee, Long pretty much decides how much we'll be taxed, how much we'll get in Social Security payments when we retire and how much we'll get in Medicare when we're sick.

Yet Long is responsible to no one except the voters of Louisiana, whose votes he controls through one of the most powerful political machines in the country.

Danforth's amendment would rule out the possibility of any senator's or representative's wielding this kind of power, at least for a considerable length of time.

It is true that time is a great teacher and that experience provides wisdom. But there are four men in the Senate presently who have been there 30 years or more. You'd think that, if they had provided the country with much wisdom, we might know them well. Yet I venture that seven out of 10 readers of this column cannot name them.

I like the remark of Sen. Malcolm Wallop (R-Wyo.), who supports Danfroth: "There's no question that, if this amendment were adopted, men of extraordinary talent would not be here. But there's equally no question that men of extraordinary talent are ready to take their places."