Democratic National Chairman Charles T. Manatt said yesterday that his party could double its gains in the Nov. 2 congressional elections if it had as much money to spend as Republicans have.

"Money alone can buy 10 to 12 seats" in the House, he told a breakfast meeting of reporters. "I think our gains will be cut in half of what they'd be if there was an equality of fund-raising."

Manatt predicted that Democrats will pick up 15 House seats in the election, which he said will be a referendum on President Reagan's economic policies.

The recent drops in interest rates and upsurge of activity on Wall Street haven't hurt Democratic prospects, he said. "The economic program hasn't worked. The economy hasn't recovered," he said. "Unemployment and bankruptcies are still high."

Moreover, Manatt accused the Federal Reserve Board of bending to political pressure in lowering the discount rate, and therefore helping to bring down interest rates. With elections around the corner, "the timing was certainly interesting," he said.

Manatt said the fact that House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and other Democrats came to Reagan's aid during last week's battle over the tax increase was a plus for Democrats because it underscored the failures of the president's economic program.

The Republican Party has built up an awesome war chest. It is prepared to spend $11 million in House races, $10 million in Senate races and more than $10 million on a national television advertising campaign. Democrats have $1 million for House races, $1.2 million for Senate races and $1 million for an ad campaign, which will run in selected cities in September.

Manatt accused Republicans of using their money to launch "a big-lie" TV campaign, and of pulling dirty tricks on Democratic candidates. For example, he said the GOP hired young people to unfurl a "Happy 81st Birthday" banner behind Sen. John C. Stennis (D-Miss.) during a campaign appearance.

Age is a major issue in Stennis' campaign against Haley Barbour, 35, a Republican.