Ronald Reagan opposes an open Democratic convention. So do some of my fellow Democrats.

I disagree.

Yet I can well understand Reagan's opposition. Whatever weakens the Democratic Party benefits him. He knows our history. He knows how our conventions have rallied Democrats to victory in the past, and he is worried that we Democrats will rally once again.

Why, then, do some Democrats favor a closed convention, a convention in which courage and conscience would be replaced by computer-like control?

Three arguments have been made.

First, we are told that to hold an open convention would be to change the rules. The argument is as misleading as it is wrong.

It is not the Rules Committee but the delegates themselves, meeting in New York, who are to make the rules that will govern the convention.

In 1974, Democrats met in convention in Kansas City. After thorough debate and deliberation, we adopted a party charter to serve as our party's constitution. Then-governor Carter and then-chairman Strauss worked hard, with many other Democrats, to shape this document and give it life. The charter is the governing instrument of our party, and it provides: "The Democratic Party shall not require a delegate to a party convention or caucus to cast a vote contrary to his or her express preference."

Two years after President Carter's election, the Democratic National Committee met, not to amend or revise the charter but to adopt new delegate selection rules. These rules were meant to ensure that the women and men chosen to fill delegate seats to our national convention did in fact support the candidates to whom they were pledged. The Winograd Commission accomplished its task superbly.

Nevertheless, among the bundle of delegate selection rules was one Rule-11-H, that dealt not with how delegates were to be selected prior to the convention but with how they would have to behave at the convention. This rule, now known as Rule F(3)(c), has been proposed by a 4-3 vote of the convention Rules Committee, as a rule to govern the behavior of delegates in New York. Rule F(3)(c) provides: "All delegates to the National Convention shall be bound to vote for the presidential candidate whom they were elected to support for at least the first convention ballot, unless released in writing by the presidential candidate. Delegates who seek to violate this rule may be replaced with an alternate of the same presidential preference by the presidential candidate or that candidate's authorized representative(s) at any time up to and including the presidential balloting at the National Convention."

This proposed rule is in plain conflict with our party's charter, with our party's tradition and with common sense itself. To say we must accept this recommendation of the Rules Committee is the same as to say we must accept without comment the platform recommended by the Platform Committee.

Five former chairpersons of our party's Reform Commission, including Morley Winograd, have joined me in urging that Rule F(3)(c) is wrong and that it should be disapproved by the delegates next Monday night. Rule F(3)(c) is the change, not our opposition to it.

Second, we are told that to hold an open convention would be a breach of faith with 19 million Democrats who participated in the delegate selection process over the first six months of 1980.

This argument is specious.

Nineteen million Democrats never voted for Rule F(3)(c). They voted instead for men and women of conscience and character. Did these voters expect the delegates to leave their consciences and characters at home when they came to New York? Did they select delegates to be pulled as puppets by the candidates and their assistants?

I do not, cannot believe it.

It is one thing for any candidate to have the right to fill delegate seats fairly won with bona fide supporters. But isn't it quite another for a candidate to have the right to dictate a delegate's vote, no matter how circumstances may have changed, regardless of the delegate's views and conscience?

I do not mean to argue that circumstances have changed since the delegates were selected two or four or six months ago. I do argue, however, that the delegates must be, and always have been, allowed and expected to decide this for themselves. New York, after all, is the first place where delegates and alternates from every state, every region and background, every complexion and heritage, finally meet for the first time. a

Do we really want a gag rule in effect? Do we really want to prevent the delegates from talking to each other about the state of our party and our country? Is this really what 19 million Democrats voted for? Is this what many millions more -- Democrats as well as Republicans and independents who look to us for hope -- expect of our party?

We are for or against no candidate. But we are against the denial to delegates of their most important right and their most important responsibility -- the choice of a presidential nominee.

Finally, we are told that to vote for an open convention and against Rule F(3)(c) would be the moral equivalent of endorsing a "dump-Carter" movement. The argument is untrue and unfair.

Among Democratic members of the House of Representatives who answered a recent questionnaire, who favor an open convention and who are publicly committed to one or another candidate, exactly one-half support President Carter and one-half support Sen. Kennedy.

No Democratic governor has been more consistent or dedicated in support of the president than has Gov. Ella Graso of Connecticut. She too, courageously supports an open convention and the defeat of Rule F(3)(c). She believes, as do so many of us, that the best and the strongest Democratic nominee will emerge from an open convention.

President Roosevelt, Truman, Kennedy, Johnson and Carter -- all were nominated at open conventions. We must nominate a Democratic president for the next four years, and once again we must do so at an open convention.