HOUSE MINORITY LEADER John J. Rhodes (R-Ariz.) has had second thoughts about putting a provision into the Constitution compelling a balanced budget. At a party council last month he opposed the amendment idea as "gimmickry"; what was needed, he said, was the political will to make tough decisions and keep spending under control. But rank-and-file House Republicans have been campaigning hard for a general anti-deficit clause, and Mr. Rhodes has been feeling the heat. Last week he endorsed the amendment concept after all.

That's just one sign of the mounting congressional pressures to get on record for budget-balancing. When the debt-ceiling increase was before the House the other day, a bipartisan coalition came very close to attaching a clause barring a deficit in fiscal 1981. Speaker Thomas P. O'Neill (D-Mass.) only turned back the anti-spending forces by two votes.

So far, signing on to this cause has been easy because no one has had to specify what spending he would cut. This grace period will run out next month when the first budget resolution comes before the House. The choices won't be easy; for instance, how would the economizers reduce the deficit below President Carter's level of roughly $29 billion, and still make room for several billion dollars in Middle East aid?

In previous years the Republicans have done what opposition parties usually do -- assailed the budget committee's extravagance, complained about parliamentary constraints, and taken vehement stands against waste. In short, they have let the majority do the hard work and then complained about the results.

Now, however, the House GOP and the Democrats clustered around Rep. James R. Jones (D-Okla.) seem to have amassed at least a near-majority. It would be nice if they started acting like one -- which would mean accepting more responsibility and making more painful choices themselves. In fact, the Republicans ought to propose their own detailed budget plan next month, so everyone can see what balances they would strike and how they would curb their own desires to spend more for this or that. For when you really get down to business, Rep. Rhodes was right the first time around. If lawmakers have enough will and self-discipline, general anti-deficit laws or amendments are not needed. And if Congress cannot curb its own sky's-the-limit impulses day by day, no general ceiling is going to hold them down.