Democratic National Chairman Charles Manatt says that his party's political gathering set for Philadelphia in two weeks will not be the miniconvention that Democrats have held in the past.
Instead, he told reporters over breakfast on Wednesday, June 9, ''it will be the first-ever national party conference. There will be no delegates. And there will be no platform.''
On other topics Mr. Manatt had this to say:
* He saw no national trend in the June 8 primaries: ''Both parties came up with strong candidates,'' he said.
* He said he thinks the Democrats will pick up 15 seats in the House and, perhaps, one or two seats in the Senate in the fall elections.
* He said he still thinks Reagan is politically formidable. ''I still - and always - say never underestimate Ronald Reagan,'' he said.
The 1,070 Democrats who will assemble in Philadelphia will be mainly members of the Democratic National Committee, together with Democratic congressional and state leaders.
Mr. Manatt stated the objectives of the conference as follows: (1) ''A chance to get a statement on Reagan - on his unfairness to many people and of his mismanagement of the federal government. Also a chance to put together a statement on the issues;'' (2) ''A chance for party leaders to be together and begin working together toward the 1982 and 1984 campaign;'' (3) ''A chance for the potential presidential candidates to meet with the party leaders.''
Speakers at the conference will include former Vice-President Walter Mondale, and Sens. Edward M. Kennedy of Massachusetts, Gary Hart of Colorado, John Glenn of Ohio, and Alan Cranston of California.