The U.S. Postal Service might be struggling financially because of a drop in mail volume, but Congress has the opposite problem. More people than ever before are writing to their lawmakers, and House and Senate offices are working overtime to keep up.

Some constituents still like to communicate the old-fashioned way — by mailing letters — but the growth of e-mail and the proliferation of Web sites enabling people to contact Congress have created a deluge of messages requiring a response, according to a report released Tuesday by the nonpartisan Congressional Management Foundation.

The improved technology and heightened interest in reaching out to Capitol Hill have run headlong into the bipartisan appetite for austerity. For nearly every congressional office, there is more work to do but not necessarily more people to handle the load.

“The question for any business or organization or culture that is experiencing that [trend] is, how do you manage it?” said Brad Fitch, the foundation’s president and chief executive.

In its report, “Communicating With Congress: How Citizen Advocacy Is Changing Mail Operations on Capitol Hill,” the foundation found that the volume of communication from constituents to congressional offices has skyrocketed in the past decade, with increases ranging from 200 to 1,000 percent over that time. The group notes that 2009 was a particularly fertile year for such communication — with high-profile stimulus and health-care bills generating plenty of reaction, for and against.

Notably, the foundation reports that the “bulk” of such messages are generated via advocacy campaigns by outside groups, and that 5,000 to 10,000 associations, nonprofit organizations and corporations have sections of their Web sites devoted to this purpose.

And the group highlights two key data points: “1) Some congressional offices have experienced a 1,000 percent increase in communications volume in the past decade; and 2) Congress has not increased staff size in personal offices since 1979.”

Since that year, the personal offices of House members have been allowed to hire only 18 full-time and four part-time workers, even though the volume of mail to offices and the size of districts have increased. After the 1980 Census, each congressional district averaged about 510,000 constituents. Once the current round of redistricting based on the 2010 Census is complete, the average district population will be more than 700,000.

And Congress is in the midst of cutting its own budget, as the House and Senate submit themselves to the same belt-tightening as the rest of the federal government. Some offices may choose to lay off staff in the coming year.

The messages Hill offices receive cover the spectrum: Some request help for a problem with Social Security or veterans benefits. Some advise a lawmaker to vote for or against a bill. And some writers — like commenters on political Web sites and callers to C-SPAN — merely want to vent on the topic of the day.

The good news is that as constituents use new technology to write to Congress more often, Hill offices are using the same technology to respond. Until recently, the foundation says, most lawmakers used traditional mail to respond, even to e-mailed queries.

Now, the group reports, 86 percent of congressional offices “answer all or most of their incoming constituent email with email, compared to only 37 percent in 2005.”

The bad news is that such responses can be a long time coming. The foundation found that “a sizable number of offices are unable to respond to constituent emails with pre-existing responses in less than a week.” And when an office does not have a pre-written response, it can take three weeks or longer to send out a reply.

Surveying congressional staffers, the foundation found that 58 percent of respondents said they spend more time handling constituent communications now than they did two years ago. Forty-six percent of aides said their offices have “shifted resources from other priorities” to deal with traditional mail and e-mail.

Beyond the volume of work, some aides told the foundation that their offices simply handled mail inefficiently — a simpler process would let them churn through more messages faster.

Despite the torrent, most staffers said they did not feel discouraged — 48 percent in the House said their offices had “sufficient resources” to handle communications, while 68 percent in the Senate said the same.

Not surprisingly, the offices that felt most comfortable using new tools also felt best able to handle constituent messages. The foundation found that a majority of “early adopters” of technology (the group released a separate report in July on congressional use of Facebook and Twitter) said their offices had the resources to handle mail, while “late adopters of technology on Capitol Hill felt much more overwhelmed.”

“The gulf between the most sophisticated offices and the least sophisticated offices is getting wider,” Fitch said.