SUDDENLY THERE ARE TWO major convention center proposals in the field, the one just unveiled by the District government and the rival plan for Prince George's County, which the Maryland House of Delegates will be voting on today. Each plan requires a larger public investment than we think is justified.

Prince George's political leaders are pulling out all available stops in Annapolis to try to persuade the House of Delegates to overrule its appropriations committee and approve a $20 million state grant for the Largo plan. The county campaign is so intense as to suggest that there may be some inadequacies, on the merits, in the plan put together hastily by developer William Levitt Jr., political leader Peter F. O'Malley and county executive Winfield M. Kelly Jr.

One cause of much suspicion is the fact that the proposed center, if successful, could be immensely profitable for Mr. Levitt and his companies by stimulating development of their land holdings in Largo. Of course, convention centers are supposed to be economic catalysts. The real question is the likelihood of success - and that's what gives us most concern. Mr. Levitt is willing to hear only limited risks, primarily the gamble that the $35-million project can be built without cost overruns. And he is apparently unwilling to try at all unless he can start at once and totally control the construction and design. The project is also chancy enough that the county, which would realize the most direct and concentrated revenue benefits, is unwilling to put up any front money and wants to limit its future obligations to a subsidy of about $1.5 million per year.

The competition from the District is of course what makes this plan even more hazardous than most. If both projects were built, the District might not benefit but Largo would surely lose. Thus one aim of the Prince George's blitz is obviously to outflank and discourage the District by rushing the plan through Annapolis, late in the session, on what might be called, if you will pardon us, a Levitt-or-leave-it basis.

This is not to say that the District's plan is fail-safe. Instead, it is even more costly and speculative. We say this with regret, because there is a need to spur new investment in the old, declining downtown area around Mount Vernon Square. Three years ago the proposed Eisenhower Center seemed to us to be a worthwhile gamble. The odds are longer now, though. The impact of inflation has been sobering. All in all, we are no longer persuaded that this project is worth the risks.

According to the current plan, a large convention center would cost from $102 million to $110 million, without parking or an auditorium, and would not obligate the city to repay between $7.6 million and $9.7 million per year in capital costs. The center, the study says, could attract from 310,300 to 390,000 new convention visitors per year, generating about $100 million in new business, mostly for hotels and restaurants, plus some 4,000 permanent jobs and new city tax revenues of $14 million or more per year. This, it is argued, would more than offset the additional debt.

Even if one accepts these generous estimates of the center's drawing power, the project's feasibility turns on three large assumptions. The largest is that construction could be completed by 1982 - and that the cost would not exceed $110 million. The second is that the project, initially financed by local borrowing from the federal treasury, could be refinanced in 1982 through District bond sales at a lower interest rate. The third is that the completed center would be smartly managed and aggressively marketed.

Each assumption, by itself, is reason enough for pause. Together, they show just how large a risk a downtown center has become. The potential burden on local taxpayers would be reduced by at least $22 million and perhaps as much as $30 million, of course, if private developers had enough confidence or commitment to do essentially what Mr. Levitt has done in mitment to do essentially what Mr. Levitt has done in Largo - assemble the site and donate it. In the absence of any such dramatic private initiative, the city's taxpayers are being asked to cover the entire bet. That means gambling not only on the city's attractiveness to future conventions, but also on the local government's capacity to carry out such a huge, complex project without any delays or slip-ups along the way.