In the tax cut proposal he made today, Ronald Reagan demonstrated that he intends to carry two banners this fall -- his own and the Republican Party's.

Unlike Jimmy Carter, who as an "outsider" campaigned against the Washington establishment, outsider Reagan intends to mesh his presidential campaign closely with the congressional wing of his own party.

As is customary in political alliances, this melding contains something both for Reagan and the members of Congress.

The GOP congressional minority, which often has trouble making its voice heard outside Washington, gained instant publicity from Reagan's advocacy of the congressional tax cut proposal. In effect, the congressmen are using Reagan's political prestige to draw attention to their own plans.

Candidate Reagan, assured of the presidential nomination when the Republicans convene in Detroit next month, obtains several benefits.

Most immediately, he demonstrates to skeptical Republican moderates that he would not run an independent, conservative presidency isolated from the opinions and the political needs of the GOP congressional wing.

Even more important for Reagan in the campaign, he has now discovered that on issues he can exploit resources that exceed his own.

Such staunch Reagan supporters as Sen. Paul Laxalt of Nevada have been urging all along that the former California governor, who often has been criticized for lacking facts and figures to back his public stands, make greater use of GOP congressional expertise.

It was with this mind that Laxalt put together a dozen congressional advisory committees which were announced by Reagan last week during a visit to Washington.

The unified tax cut proposal announced today is likely to be a precedent for similar alliances on defense and energy issures where the congressional minority has considerably more expertise than does the Reagan campaign.

Reagan habitually criticizes the Carter administration but is unable or unwilling to say exactly what he would do as president. For instance, he has yet to say what weapons systems he favors to bolster the strategic deterrent, or to put a price tag on his various proposals for upgrading the armed services.

The success, therefore, of the honey-moon between the Reagan campaign and the congressional wing is likely to depend on two factors -- whether Reagan can accept, as he did today, a specific legislative remedy to a problem he has described, and whether the congressional wing itself stays unified on major issues.

But for now, at least, the mutual dependence of Reagan and the congressional wing is viewed as an unmitigated asset.

"Reagan's views are well known and no one thinks he is going to be the captain of any congressman," said a Reagan aide today. "But he is not an independent candidate. He is the candidate of his party, who intends to work for other Republican candidates as he did in California and who intends to work closely with his supporters in Congress if he is elected president."

The timing of today's announcement was closely synchronized between Reagan operatives and the GOP congressional leadership.

Originally, the announcement had been planned for Thursday. But when Reagan was told that today would be more convenient, so that it could be introduced Thursday, he went along even though it meant shortening vacation time on his remote ranch north of Santa Barbara.

Instead of spending Tuesday night at the ranch, he returned to Los Angeles to prepare for this morning's news conference.

Press secretary Ed Gray said that negotiations on the timing and content of the announcement had been underway for more than a week. Judging by the satisfaction here with the joint announcement, such negotiations could well become a regular feature of the Reagan campaign.