BRENT COLES, mayor of Boise, Idaho, is getting an earful about buses. Since late August, when he announced that congressionally proposed federal budget cuts could force his city to raise bus fares by a third, local citizens have been angry. ''My hotline has been ringing off the hook,'' Mr. Coles says. This week, he and hundreds of other mayors, county executives, and municipal leaders around the country will hold town meetings, press conferences, and public hearings to raise local consciousness about how Washington's plans to reduce federal funding will affect city services - from streets and school maintenance to sewage treatment. In response to a survey conducted by the United States Conference of Mayors, city officials listed areas that will be hardest hit if the budget cuts approved by the House and now before the Senate, are enacted: mass transit, crime prevention, Head Start, and job training, among them. More than 40 percent of the officials say they would raise taxes and some 80 percent expect to reduce city services. ''Most Americans don't have a clue as to what these proposed budget cuts will mean,'' says Jerry Abramson, mayor of Louisville, Ky. Mayors are uniquely qualified to drive the message home, he adds. ''We're the ones who have the most credibility with our constituents - we have to decide when and where to expend funds. And we're very uncomfortable with politicians in Washington who have been far removed from the local street level for many years determining what's in our local interest.'' Coles's problem in Boise typifies what urban leaders nationwide face - a withdrawal of Uncle Sam's support that is difficult to replace with taxpayer revenue. The budget cuts would hit his $1.5 million transportation budget hard - $800,000 of it is federally funded. Lawmakers want to slash that number by $480,000 in the coming year. Eventually, Coles says, they aim to totally phase out federal support. State law prohibits the mayor from raising property taxes any higher to help pay for the 30 buses that provide 1 million rides a year. So unless the city charges more for public transport, Coles will have to slash other city services, which are already ''bare bones,'' he says. In many places - from Louisville's population of 1 million to Lincoln, Neb.'s 200,000 - local taxpayers are tapped out. Raising revenues to finance Louisville's transportation costs, social service needs, and antidrug campaigns in public housing projects - all of which are slated for big cuts in the proposed budget - is simply not an option, Abramson insists. And don't look for charities to make up the shortfall, he says. Smaller cities like Lincoln simply don't have the individual and corporate tax base to finance their own budgets and meet Washington's mandates, says Mayor Mike Johanns. The high cost of complying with federally imposed environmental, disability, and other regulations is passed on to taxpayers who are already groaning, he says. Urban officials are looking to Congress to revamp its spending priorities. COLES, a Republican, says he is willing to support the fiscal austerity in Boise so long as outlays for defense, medicare, and other federally funded programs are ''taking that kind of cut.'' Mr. Johanns, also a Republican, is frustrated ''that the Contract With America is stalled.'' If it doesn't move as package [of budget cuts and mandate relief], we could end up with all of the cuts and none of the relief. We'd kind of be living with old Washington with none of the benefits of new Washington.'' Many Democrats call for more, not less, federal financing. ''We should also reexamine the largest federal program of all,'' urges Mark Green, public advocate for New York City. Federal budgeteers have ''drained our urban areas of funding and put the defense budget over workfare and economic growth.'' Slash the defense budget by a third and pour the savings into cities, he says. ''Disease, crime, poverty, and ignorance are surely greater threats to our nation and our people than any foreign army.'' In the coming days, Seattle Mayor Norman Rice, president of the US Conference of Mayors, expects to stir up resentment among Americans who have not thought much about how the proposed budget cuts will affect them. ''The new Congress went to Washington promising reforms, but all they've done is pass the buck.... There's not much partisanship among mayors,'' Mr. Rice says. ''Every mayor across this nation is going to feel these cuts.'' House Speaker Newt Gingrich (R) of Georgia ''has asked us for a list of areas in the federal government that make it more difficult for us to do our job,'' says Louisville Mayor Abramson. He complains that when it comes to how the Republican leadership's budgeteers will affect the cities, ''we haven't had an opportunity to make our thoughts heard. They've given us their fax numbers and their 800-numbers, but they told us that they weren't holding any hearings.'' Lawmakers who just returned to Washington this week after their August recess can expect to hear from the mayors, Coles says. ''We're concerned that maybe our voice hasn't been strong enough. Some would say we've been ignored.''