Senate backers of the MX missile responded to a debating challenge from its foes yesterday and, while conceding the vulnerability of the weapon's basing system, described the MX as "essential" to progress on arms control.

The brief exchange came as the Republican-controlled Senate prepared to vote today on authorizing $2.5 billion in production funds for the first 27 missiles, with some leading MX critics conceding in advance that they will lose, at least in this round.

The money may be authorized, but "it ain't been appropriated yet," said Sen. Dale Bumpers (D-Ark.), promising, by implication, a renewed fight over the MX when funding for the missile comes up for approval this fall.

Majority Whip Ted Stevens (R-Alaska) predicted that the MX authorization would get 55 to 60 votes in the 100-member Senate, which would mean a bigger margin than the House provided last week when it approved a similar MX authorization by a vote of 220 to 207.

The MX issue finally was joined after Democratic presidential candidate Sen. Gary Hart (D-Colo.), who has been leading the charge against the missile, formally introduced his amendment to strip MX production funds from the $200 billion defense authorization bill that has been pending before the Senate for two weeks.

Hart contended that the MX is so vulnerable to attack that it would force the country to choose between a policy of "attack-on-warning" or development of an "expensive . . . problematic . . . and probably enormously destablizing" anti-ballistic missile defense system.

Such a defensive system could add between $30 billion and $40 billion to the cost of the entire MX program, doubling its cost, Hart argued.

"This decision isn't going to stay behind us; it is going to keep coming back . . . ," Hart said. "It the MX has no internal logic, it has no constituency, it has no means of self-justification."

In response, Armed Services Committee Chairman John G. Tower (R-Tex.) characterized the MX and its deployment plan as a bipartisan compromise that amounted to "the best effort to get something that was militarily acceptable and at the same time politically possible."

Conceding that the missile's planned deployment in existing Minuteman missile silos makes the huge 10-warhead weapons vulnerable to Soviet attack, Tower said the United States doesn't "seem to have much in the way of a timely alternative."

Although vulnerable, the MX is "essential if we're to make any satisfactory progress" in arms-control negotiations with the Soviets, said Tower, describing it as "negotiating leverage" to force concessions at the bargaining table.

Rejecting the notion that the MX would help win an arms agreement, Bumpers said it would result only in "a hundred vulnerable missiles in a hundred vulnerable silos" and possibly a defense system that would bring the United States "to the brink" of violating the anti-ballistic missile treaty with the Soviets.

"We just don't know what else to do with the darn thing," added Sen. Carl Levin (D-Mich.).

Hart's amendment to strip MX production funds from the defense bill is one of four MX-related amendments that the Senate is expected to consider today before voting, probably tonight, on final passage of the bill.