Congress is so concerned about President Reagan's sprint to a strategic missile treaty by next spring that it may be ready to do what until now has been undoable: finance a long-range mobile missile to protect the United States against surprise Soviet attack.

Reagan's zest for a Moscow summit next June to sign a START treaty cutting long-range missiles by 50 percent has the congressional defense bloc in a frenzy. The huge advantage of Moscow's new array of mobile missiles would double the threat of a successful surprise attack's knocking out all U.S. land-based missiles, none of which are now mobile.

Here is an unexpected byproduct of the move toward De'tente II. Reagan refused to admit to Mikhail Gorbachev last week that he is ready to accept mobile missiles as part of either superpower's arsenal, but Congress is quietly starting to do just that. That means either converting the silo-based MX into a mobile missile or finally going ahead with the mobile Midgetman.

At work here are strange congressional bedfellows. Sen. Albert Gore Jr., the Democratic presidential candidate seeking to stake out a defense constituency, last week said he would vote for at least 50 rail-mobile MXs, the big missile that carries 10 warheads. Defense-oriented Republicans such as Sen. Pete Wilson applauded Gore for going public on an issue opposed by most of his party. But Wilson and other worried Republicans want more than that. Their goal is 100 mobile MXs, and they think they will get there.

A tip-off was the success of Armed Services Committee Chairman Sam Nunn in getting the Senate to vote early financing for the Midgetman last week. He was furious over an earlier Senate decision to kill Midgetman as part of a catchall spending bill that bypassed his committee.

Amid this activity, the president strangely clings to a policy that the United States will not allow either side to build mobile missiles because agreements governing them cannot be verified. At one point in summit talks with Gorbachev, Reagan was pushed hard to approach the Soviet boss with this hypothetical question: If the United States did endorse mobile missiles, how much on-site inspection would you give us to make sure you don't cheat?

Even that small step away from the official U.S. position that mobile missiles are a no-no was ruled out. Instead, Reagan's arms-controllers say privately they will ''switch on mobiles'' only when they are sure to get ''the best concessions'' that can be had from the Soviets.

But in Congress the question is becoming blunt and persistent: How can the United States expect ''concessions'' from Gorbachev on mobile missiles? The Soviets have been building them all through the Reagan years, while the president and his administration went first this way and then that way on mobile missiles -- the MX and the Midgetman -- and ended up with nothing.

Realists in the administration agree it is preposterous to think the Soviets would pay anything of substance for a U.S. go-ahead on mobile missiles. They already have deployed what U.S. experts estimate to be 120 mobile SS-25s, a virtual carbon copy of the Midgetman, and 30 mobile SS-24s, a missile very similar to the MX. Their target is estimated to be 500 SS-25s and at least 125 SS-24s.

Add to this the mainstay of their land-based long-range force, the silo-based SS-18, and the Soviets possess an awesome array of power. It would not be adversely affected by the proposed 50 percent cut in Reagan's START treaty.

Indeed, what would occur under START would be precisely the opposite. A confidential memorandum written by one of the president's longtime arms-control advisers puts the matter bluntly: Soviet ability to wipe out the U.S. land-based force of intercontinental missiles would just about double after the 50 percent START reductions had taken place.

A major reason for that stark computation is the fact that all U.S. missiles are in silos. But while the Soviet missile-kill ratio against U.S. targets would double, the comparable U.S. ratio against Soviet targets would remain about the same after all long-range missiles were reduced by half. Again, the reason for this relative Soviet advantage is missile mobility.

That explains what seems to be in the works on Capitol Hill. As Reagan moves toward Moscow on a dead run for his START treaty, Congress is beginning to understand this country will face terrible risks after that treaty has been signed and long-range missile warheads are chopped out of the arsenal, one by one.

But that real terror has not yet being fully and openly talked about by Reagan's arms-control men. The question now is how soon the sprint toward START will be accompanied by a dash toward protecting the nation's security.