The Senate yesterday joined the House in signaling opposition to President Reagan's Persian Gulf policy with a 56-to-42 vote that kept alive a proposal to force a 90-day delay of his plan to reflag 11 Kuwaiti tankers and give them U.S. naval protection.

But the Senate stopped short of passing the proposal, as foes of the reflagging operation from both sides of the aisle failed to muster the 60 votes necessary to break a threatened Republican filibuster against the delay measure.

The delay was offered as an amendment to trade legislation that Senate Democratic leaders are struggling to pass within the next few days, and a filibuster on the Persian Gulf issue would have further complicated passage of the trade measure, a top-priority item on the Democratic agenda.

House and Senate leaders conceded some time ago that congressional action would be largely symbolic because Congress lacks the time and votes to override a presidential veto of any legislation that would attempt to block the reflagging, scheduled to take effect within the next week or so.

But the Senate action, coupled with a 222-to-184 House vote Wednesday approving legislation to force a 90-day delay, puts new pressure on the administration to reassess the reflagging and escort plan before it is implemented, foes of the operation say.

"I think the president will be inviting {congressional} leaders down to the White House and asking how we can resolve this problem," said Sen. Dale Bumpers (D-Ark.), who, with Sen. Mark O. Hatfield (R-Ore.), led the push for Senate approval of the 90-day delay.

The vote -- and threatened complications for the trade legislation if the controversy is not resolved -- also appeared to put pressure on Democratic and Republican leaders to reach a compromise on language that could get through the Senate without a filibuster.

Yesterday's Senate action opened with an unsuccessful effort to thwart a threatened filibuster against an earlier proposal that called on the president to hold the reflagging "in abeyance" to allow time to develop other strategies for ending the Iran-Iraq war and protecting the flow of oil through the gulf.

The vote on this nonbinding proposal, offered by Sen. Daniel P. Moynihan (D-N.Y.) and supported by Majority Leader Robert C. Byrd (D-W.Va.), was 57 to 42, three short of the needed 60 votes. But Democrats got more votes than they had anticipated, and Byrd said he plans another attempt to break the filibuster next Tuesday.

Bumpers and Hatfield then forced a vote on their proposal by using the procedural ploy of a tabling motion, which was defeated largely along party lines. All but three Democrats voted against killing the proposal, while seven Republicans broke party ranks to support it. Among the Democrats who voted to table the proposal was presidential candidate Albert Gore Jr. (Tenn.).

In debate, reflagging foes cited the Iranian grenade attack on the U.S.-owned tanker Peconic in the Gulf yesterday as an example of what Sen. James R. Sasser (D-Tenn.) called a policy "charade." Kuwaiti-owned tankers will have U.S. protection, while American-owned vessels sail there unprotected, he said, calling this the "illogic of administration policy."

But Sen. John W. Warner (R-Va.) said a congressional vote against reflagging would "denigrate" U.S. credibility in the Middle East and "strip . . . the president of his epaulets in front of the world."

Meanwhile, White House spokesman Marlin Fitzwater said Wednesday's House vote "represents the kind of sidelines diplomacy that hurts U.S. foreign policy." House members who voted for the delay "want to stand on the sidelines with a back-pocket press release that says, 'We told you so,' but they don't want to go out and take responsibility."