While Mark A. Siegel's split with President Carter's even-handed Middle East policy was probably inevitable, a troubling question persists: Why must this talented young man now be entirely lost to a beleaguered White House in desperate need of talent?

The indistinct answers probe into murky corners of life in the Carter administration: continued suspicion of outsiders by the president's campaign operatives; an ambiguous role played by Vice President Walter Mondale's office; the Democratic Party's old "regulars" v. "reformers" feuding carried on within the walls of the White House.

Wholly apart from the Mideast, Siegel's loss ought to be cause for White House mourning; instead, the enemies he has there have been congratulating themselves that the wicked witch is dead. That is ironic because Siegel has been more devoted to Jimmy Carter's real political interests than some of Siegel's foes on the vice president's staff. So, a few thoughtful Carter insiders, while disagreeing with Siegel's emotional Mideast position, see in his departure a poor reward for loyalty.

But unfounded suspicion about Siegel's loyalty has plagued him since Jan. 21, 1977, when he entered the White House as an assistant to Hamilton Jordan. As executive director at the Democratic National Committee, Siegel had been part of the Washington establishment, a key strategist for the party "regulars," and a protege of Hubert Humphrey who urged the senator to run against Carter in 1976.

Nor was that all. Siegel knew entirely too many reporters, nearly as bad a stigma at the White House now as in Nixon days. Press Secretary Jody Powell unjustly suspected Siegel of being a source of embarrassing leaks. Carter campaign veterans, led by appointments secretary Tim Kraft, refused to accept him.

Early last year, a reporter who had written critically of the Carter administration (and had not seen Siegel in weeks) bumped into Kraft in a corridor of the Executive Office Building next to the White House. "Can't you find Mark Siegel's office?" Kraft asked sarcastically. Like Nixonian predecessors, Siegel feared being seen in restaurants with newsmen not on the Carter friends list.

In truth, Siegel was scrupulously loyal to the president in word and deed. Furthermore, his political contacts were a rare and essential commodity in the Carter White House - an underutilized commodity. Early in 1977, Siegel was assigned the secondary and frustrating task of shepherding the Equal Rights Amendment (ERA) through state legislatures.

Gradually, however, he grew closer to Jordan, who convinced Powell of his dependability. Siegel was given the task of protecting the president's interests at the Democratic National Committee. They needed protection because of the unfortunate choice as Democratic national chairman of former Gov. Kenneth Curtis of Maine, who viewed big-time party politics as a New England town meeting.

This brought Siegel into confrontation with Richard Moe, the vice president's formidable assistant and an ally of the national committee's "reform" element. While Siegel was carrying out Jordan's desire for rules changes to make more difficult a 1980 insurgent challenge against Carter, Moe was sympathizing with "reformers" and Curtis in their crusade for internal democracy within the party.

Curtis was eased out as national chairman, but it was a Pyrrhic victory for Siegel. Angry "reform" members of the national committee were placated by Moe's assurances that Siegel no longer would handle party affairs for the White House.

Siegel was delighted last autumn when he became White House emissary to the American Jewish community. In fact, however, considering Carter's courageous peace policy in the Mideast and Siegel's sincere personal commitment to Zionism, trouble was inevitable.

When Siegel went to Jordan March 1 and told him he could no longer defend the Carter Mideast policy to the Jewish community, his intent was to stay on at the White House. Siegel's enemies are spreading the word that Jordan and the president wanted him out entirely. Siegel declared unequivocally that the decision to resign was entirely his own.

Assuming that Siegel jumped instead of being pushed does not change matters much. The political background doomed Siegel once he opposed the president on the Mideast. The immediate reward for one year's loyal service in a variety of unpleasant tasks was Carter's uncongenial reference to him over national television as "an employee." Among some thoughtful souls at the White House, that is cause for sadness and concern.