So a former vice president from the Midwest endorses a political friend who's running for mayor of a Midwestern city. It may not do much for the mayoral candidate but, as they say, it couldn't hurt.

Couldn't it? You ought to listen to Jesse L. Jackson, head of the Chicago- based Operation PUSH, who describes presidential hopeful Walter Mondale's endorsement of Richard Daley Jr. in terms that include "betrayal," "hypocritical and contradictory," "dismaying," "insensitive" and, in a lighter vein, "dumb, just plain dumb."

What raises the ire of Jackson, other black Chicagoans and black political leaders across the country is that the endorsement comes in the closing days of a race in which a black candidate is rated to have a good chance of upsetting incumbent Mayor Jane Byrne.

Rep. Harold Washington, a Chicago Democrat, announced for the post just after winning reelection to Congress last November and, according to some polls, is currently in second place: behind Byrne but ahead of Daley. Chicago blacks, long unhappy with Byrne's administration, have pressed a registration drive that has pushed their strength to around 700,000, an electoral base Washington had hoped to tap. The fear is that the Mondale endorsement could drain off enough of the anti-Byrne vote to defeat Washington in the Feb. 22 primary.

Last week, Jackson sent Mondale a Mailgram (signed by several members of the Black Congressional Caucus and other black civic, labor and political leaders) charging the former vice president with "profound disrespect" and warning that the endorsement, by alienating blacks, could be "a critical factor in your own stated ambitions."

"You have enjoyed great support from the black community," they said, "and for you to endorse a local white politician over a member of the Congressional Black Caucus forces us to seriously (re)consider your judgment and sensitivity."

Was Jackson saying that it is illegitimate for a white liberal presidential candidate from Minneapolis to endorse a white man for mayor of Chicago? Or that it becomes illegitimate only if there is a black candidate in the race? Or that it is legitimate for blacks to support black candidates but racist (or at least illiberal) for whites to support white candidates?

"Realistically," Jackson said, "we have to recognize that racism reduces our options to the point that we cannot hope to be elected president or senator or governor. Therefore, we have to increase black political participation where we can. You would think that a liberal like Mondale would understand and appreciate this."

Mondale says he does. "I'm an old friend of Rich Daley's, and I promised him, long before Harold got into it, that I would help him," he said in a telephone interview last week. "It's the first time I've been in a race when there's a black candidate on the other side, but I made a commitment, and I believe, in politics as in anything else, you keep your word."

Jackson, unmollified, insists that Mondale's endorsement of Daley is not just a betrayal of liberalism but also a lapse of political judgment.

The most Mondale could hope to gain from endorsing Daley would be a few thousand Daley-delivered votes in the presidential race, Jackson reasons. If, on the other hand, Mondale had endorsed Washington, it would have increased his popularity with blacks across America. He described Mondale's decision as "dumb, just plain dumb."

As a matter of fact, it's hard to know just what, if any, effect the endorsement will have on Mondale's presidential ambitions. If Daley wins, there could be a reservoir of black bitterness. But what if Daley, with the help of the Mondale endorsement, takes enough votes away from Byrne to elect Washington? And if Byrne wins reelection, will anybody remember whom Mondale endorsed last week?