As the Democratic National Convention moved into high gear Monday, the Kerry campaign was wrestling with mounting frustration among Members, key donors and lobbyists over the humble piece of glossy paper that unlocks access at the Fleet Center: the credential.

“There are definitely complaints,” said a senior Democratic Congressional aide. “But people have been concerned [for some weeks]. They were concerned then, and they are concerned now.”

Rumors swirled about thousands of Kerry supporters jockeying, unsuccessfully, to get into the convention. And some Congressional staffers were apparently losing their floor privileges so that donors to the Kerry-Edwards campaign could be present at the events celebrating the presidential hopes of the Massachusetts Democrat.

One well-placed lobbyist said that some key donors are “going ballistic” over the credential problem. The lobbyist added that the Democratic National Committee had originally promised each donor four credentials, then whittled that down to two.

Married donors who received two credentials per couple in some cases were given two different types of credentials that carry two different levels of access. One type is an “honored guest” pass, the other is a lesser “special” pass.

The Kerry camp responded that the problem is not their fault — and not as bad as some were claiming. They did add that the Fleet Center is smaller than many of the venues that have hosted national conventions in the recent past.

DNC officials did not return phone calls Monday.

Both the Staples Center, which hosted the Democrats’ 2000 convention in Los Angeles, and the Fleet Center can seat 18,000 to 20,000 people, depending on the event. But the Staples Center is approximately 200,000 square feet bigger, and some of that square footage was devoted to extra general-admission credentials.

“The space here is one-third smaller than in 2000,” said a Kerry campaign official.

So some of the estimated 35,000 delegates, journalists, foreign dignitaries and guests scrambling to get into this year’s big party were grumbling.

Several sources said the criticism is widespread among Members of Congress, with many continuing their attempts to rustle up Fleet Center passes as late as Monday afternoon.

As recently as two weeks ago, sources said, the Kerry campaign held back on releasing a share of convention floor credentials for key donors. One source said that while it is typical for the presidential campaign to reserve a number of credentials, the number was “higher than normal” for this convention.

Meanwhile, Members and aides were clamoring for extra floor passes for the biggest nighttime speeches. Several senior-level Democratic aides said that floor credentials for former President Bill Clinton’s address Monday night were in highest demand, followed by vice presidential candidate Sen. John Edwards’ (N.C.) speech Wednesday and, in third place, Kerry’s acceptance speech on Thursday.

Indeed, while Kerry’s address will arguably be the highlight of his career, Congressional aides speculated that it won’t be the biggest draw of the convention, given Clinton’s longstanding appeal among the party faithful and Edwards’ newcomer status to the ticket.

“The fact is President Clinton is one of the most popular presidents ever — certainly among Democrats — and one of the most popular political figures in the country,” explained one House Democratic leadership aide. “John Kerry won’t do away with that, no matter how much energy he generates.”

Mary Ann Akers contributed to this report.