The Democrats handed President Reagan the peace issue on a silver platter.

It is the only one they have, now that the economy is improving, but with their eyes wide open 91 of them, including four members of the leadership, joined the Republicans in the House to give the President a blank check on arms control.

After the MX vote, Rep. Tony Coelho (D-Calif.), chairman of the Democratic Congressional Campaign Committee, who voted against it, worried, "It raises questions about where we are as a party."

Rep. Thomas J. Downey (D-N.Y.), the boyish leader of the MX opponents, fumed:

"If there was a difference between Republicans and Democrats, it was this. If Democrats aren't for arms control, who is?"

The House chamber rang with Democratic doublespeak, as people who three weeks ago voted overwhelmingly for the nuclear freeze explained that their assent to the deadliest weapon yet devised did not mean they cared less about ending the arms race. Nor did their reversal from last December, when they turned down the MX, mean that they had changed. It is Reagan who has changed.

Reagan, they said, has taken the pledge. He just needs one more for the road as he heads for the table in Geneva. Although the president's rhetoric--the "Star Wars" and "Evil Empire" speeches--raised doubts elsewhere, theirs were laid to rest by a letter he wrote to them.

They were not so much voting for the MX as they were for "a package," for "bipartisanship" and for the Scowcroft Commission, an assemblage of hawks who reported that while the MX is indefensible we must build it because Reagan wants it.

Rep. Albert Gore Jr. (D-Tenn.), one of the three key Democratic defectors, said that Reagan is the only peacemaker we have.

Not since the Gulf of Tonkin resolution has a president enjoyed such a triumph for such a dubious position. The Republicans sounded several Vietnam chords--they talked of the need to "negotiate from strength," of "not tying" the president's hands, of keeping "unpredictability" in dealings with an adversary. The Democrats were deaf to these strains from the years that cost them so dear.

Not since Lyndon B. Johnson has Washington seen a more masterly presidential manipulation of congressional egos and conflicting desires.

Reagan played brilliantly on House members' profound need for notice. They are so many, and they feel left out. Reagan elevated the orphans to the level of policy-makers. He gave them something headier, the impression that they had turned him around.

Rep. Les AuCoin (D-Ore.), one of the unconverted, complained that the congressional negotiators with the White House had brought back no bargain.

"The president gets the MX and Congress gets a statement of sincerity about arms control" he observed dryly.

What the Democrats gave Reagan beyond a new super-weapon was a reminder of his old political super-weapon, the incurable fondness of the American people. Unspoken through the debate was the Democrats' deep fear of opposing a popular president.

The Democrats are buffaloed by Reagan's continuing, increasing hold on the the country's affections. People say that they don't like his policies--they hate the arms race, for instance--but they like him.

It seems not to have occurred to the Democrats that the reason people like Reagan might be that, unlike them, he knows exactly what he wants. The need to be "neo," to be almost-but-not-quite, that afflicts so many liberals stops short of the White House. Reagan says, "I want--and no ifs, ands or buts about it."

Democrats may protest a secret CIA war in Central America. But when it comes to stopping it, they lose their nerve. Reagan's people say that bringing the violence to a halt would be "damaging."

The damage being done was vividly detailed by Washington Post correspondent Karen DeYoung, whose tax dollars came back to her in the shower of hot lead that rained around her when her escorted Jeep was ambushed on a remote road in Nicaragua.

But the Democrats, who think it is wrong and know it is breaking a law they passed forbidding such mischief, are having second thoughts. They are willing to negotiate with the war-planners in the administration.

It makes you wonder why the Republicans are so laboriously trying to sabotage the Democrats' fund-raising telethon. Under the circumstances, with the party saying "yes--but" on arms control and "no--but" on secret wars, any external effort to make them look bad seems excessive.