About 300 mayors begin the 46th annual convention of the U.S. Conference of Mayors here today amid signs that their organization's once formidable political clout has declined.

Item: President Carter and Ambassador to the United Nations Andrew Young, the majors' first choices as main speakers, sent regrets.

Item: The conference got some unwanted publicity when seven majors declined to attend the convention because it is being held in Georgia, one of the 15 states that have not approved the Equal Rights Amendment.

Item: In January, just after Carter announced his new budget, Detroit Major Coleman Young said the President had done "very, very well" on the urban issue while, at the same moment, Syracuse Mayor Lee Alexander was saying that mayors all over thw country were "alarmed over the economic philosophy reflected in Carter's budget.

Item: A few hours before Carter announced hi program in March, the conference criticized it for not giving enough attention to employment, housing and transportation problems. Minutes after Carter spoke, several mayors anointed the program as the "ten commandments for urban America."

All this has given at least the impression that the mayors can't agree on major issues and, by asking for more money to solve every problem, have lost their audience among government officials.

Acknowledging that the organization was caught "off balance" by Carter's urban program, the conference's executive director, John J. Gunther, said recently, "We didn't realize how much we had won. We don't know how to accept victory."

But as the mayors assemble today, they are worried less about their organizational clout and more about whether Congress will look kindly on Carter's urban program. They fear that tax-cutting Proposition 13 and official preoccupation with other issues may submerge the 15 measures that make up the package.

The most optimistic guess of conference lobbyists here is that Carter's proposals for public work maintenance jobs, money that will replace the expiring antirecession aid to cities and some programs to help neighborhoods, are about that Congress will pass this year.

These lobbists do not hold much hope for other parts of the Carter package -- incentives to states that help cities, tax credits for businesses that hire long-time unemployed youths or invest in depressed areas or a so-called national development bank that would guarantee loans to businesses building plants in such places.

One reason those programs are in trouble is that many members of Congress are not convinced they will do much good. Another reason is that they are late in coming from the White House to Congree -- indeed, the bank proposal is still being written.

Finally, as one of lobbyist sympathetic to the cities put it, "the political mood has changed. People don't care much about cities any more."

One administration official said big city mayors "are out of step with the current mainstream of thought -- that curbing inflation is the nation's highest priority.

He added that California's vote for Proposition 13, a measure that slashes local property taxes and limits the state's ability to raise other taxes, " has scared a lot of people in Washington."

And, he noted, "big-city mayors represent a smaller percentage of the country."

The nation's population shift from cities to suburbs is a key factor in problems that the conference, which represents 550 cities over 30,000 population, may face in persuading Congress to provide new money to cities.

While 101 million people still live in those cities over 30,000, only 107 House members -- fewer than a quarter -- come from districts in which at least half the population lives in a central city.

That is a major obstacle, Gunther concedes. "The affinity between city and suburb is not as great as between one city and another city," he said.

So, to get new money for cities, municipal lobbyists have to enlist support from suburban members of Congress. Some congressional sources predict that the suburbanites may feel that by voting long-term loan guarantees for New York, they have already done their bit for urban America this year.

All this is not to suggest that big-city mayors and their conference are going the way of the brontosaurus. It is not to suggest that they cannot repeat such legislative victories as they had with revenue sharing in 1972 and antirecession aid for cities in 1975.

They are still a major force in American politics and as the next presidential election approaches will be even more important because of their influence on big-city voters.

One administration official who calls himself "an old mayor-watcher," says the mayors -- despite the status of the urban program in Congress -- still have influence. He noted that Carter kept his campaign promise to initiate an urban policy and told administration agencies not to propose new programs without considering their impact on cities.

"That alone could have a penetration on domestic policy second only to war," said the official, Lawrence O. Houstoun, acting deputy secretary of commerce.

Anothe source says the Conference of Mayors still has the entree to the White House and to congressional leaders that other governmental lobbying organizations do not have. He adds, "If youw ant media attention on a domestic issue, you get it by asking a big-city mayor to testify. If you ask most governors or any county officials, you get no attention at all."

Perhaps the issue of the major speakers at this convention shows that the conference can still show political muscle when it has to.

In February, conference officials asked the White House for Carter and Young. After months of phone calls, they were told no in early May. At that point the conference's deputy executive director, J. Thomas Cochran, told Carter's newest troubleshooter, Anne Wexler, that he was "going to the Hill" for a speaker.

He invited Sen. Edward M. Kennedy (D-Mass.), who was then shown in the polls as beating any other candidate for the Democratic presidential race nomination in 1980. In 48 hours Kennedy accepted, and conference official quickly relayed the news to the White House.