More than 13 months before next year's congressional elections, the Republican National Committee yesterday launched what it expects will be the longest and most expensive national advertising campaign ever put together by a political party.

Party leaders said that the ads, all singing the praises of the Republican tax cuts, were being aired so early in part to defuse criticism that has set in this fall of the Reagan economic program.

"We know we're going to be demagogued by the opposition and we felt it was important to get out there early," Richard Richards, chairman of the RNC, said at a press conference at the Capitol.

The cost of the advertising, expected to exceed $9 million, will be shared by the RNC, the National Republican Congressional Committee and the National Republican Senatorial Committee.

Last year the same three groups joined forces to produce "Vote Republican. For a change" ads that were negative in tone. This year, with their party in power, the GOP leaders have expanded the tag line to read, "Republicans. Leadership that works. For a change." This time, the theme is rigorously upbeat.

One ad, for example, shows a man getting his weekly paycheck and discovering it is for more money than usual. He tries to tell the payroll person there's been a mistake, but a voice-over reminds him of the tax cuts. He and his coworkers in the payroll line promptly break out into ear-to-ear grins.

"Since this is the first week people are going to see the cut reflected in their paycheck, we thought it was appropriate to remind them that this is a Republican tax cut," said Rep. Guy Vander Jagt (R-Mich.), chairman of the congressional committee.

During the fight over his economic legislation this spring, President Reagan held out the promise to all Democrats who voted with him that he would not actively oppose their reelection next year.

But Vander Jagt made it plain his committee will not be constrained by that promise. He said the ads would be aired against all Democrats.

"Our goal is to take control of the House next year," he said, and went on to predict that, if inflation and interest rates "come down a bit by next summer," that goal will be reached.

The first wave of the ads will be shown this week into early November, at a cost of $2.3 million. From then on, the campaign will be updated as circumstances dictate, but they will be shown continuously through November of 1982 except for some down-time around the Christmas holidays.

Last year, the Republican advertising campaign, best remembered for its use of an actor who strongly resembled House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), did not get under way until nine months before the election.

The Democrats have no comparable ad campaign planned, primarily because they lack the resources. Midyear Federal Election Commission reports issued by the three major Democratic campaign committees showed contributions totalling $3.6 million; the three Republican committees had raised $42.2 million.

A Democratic National Committeee spokesman said yesterday that the party had originally hoped to conduct a nationwide ad campaign, but has scrapped that proposal in favor of targeting ads in key districts. The Democrats have tentatively budgeted $2.3 million for that campaign, which is not expected to gear up until next summer or fall.