WASHINGTON — Larisa Thomason is a diehard liberal in a deep-red state, running a feisty Alabama political blog and donating thousands of dollars over the years to President Barack Obama and other Democrats.

But two weeks ago, after opening yet another desperate fundraising e-mail from Sen. Sherrod Brown (D-Ohio), Thomason decided she’d had enough: She clicked “unsubscribe.”

“It was a Saturday or Sunday afternoon and I thought, ‘Good heavens, I already got two e-mails from him today,’ ” said Thomason, political editor of the “Left In Alabama” Web site. “You have to think it must work, and they’re just trying to break through all the noise. But it’s just too much to take after a while.”

Eager to gain advantage in a tight and expensive election year, political campaigns are drowning their most ardent supporters in a deluge of messages begging for cash. The pleas from Obama, Republican challenger Mitt Romney and a host of other political figures are at turns cajoling, intimate and sometimes downright panicked — all aimed at squeezing another $3 or $300 out of loyal donors.

“Nightmare,” read the subject line to one missive sent last weekend by the Democratic Congressional Campaign Committee, warning that Obama could lose. “If we don’t fight back, it’ll be over long before November.”

The push for donations is easier than ever by the the ubiquity of e-mail, social media and texting services, all of which can be ruthlessly exploited for fundraising pitches. Both major presidential candidates are aggressively using such technology to shape their messages and operations, including new iPhone apps released last week aimed at getting supporters more involved in their campaigns.

But political strategists from both parties warn that campaigns must be careful to avoid alienating their most devoted followers with an endless tide of fundraising e-mails. Almost by definition, those who receive the most donation requests have also given to the campaigns already and, in many cases, are annoyed at being pestered for even more.

“The question really isn’t whether or not an aggressive e-mail plan will produce money, because almost all fundraising e-mails will,” said Austin James, vice president for digital strategy at Gridiron Communications, a Republican consulting firm. “It is about the frequency and tone with which you ask for money that plays into the overall health of your list and perception of your campaign.”

In trolling for cash, nothing approaches the sheer assertiveness of the Obama campaign, which had 13 million names on its initial e-mail list and has pushed into overdrive after falling behind Romney in monthly fundraising. The Romney campaign and its affiliates raised $183 million in May and June, compared with $131 million for Obama’s operation.

ProPublica, the nonprofit investigative journalism Web site, has collected more than 600 separate fundraising e-mails from the Obama campaign this year, nearly all of them sent in the past three months. The group has tallied about 100 similar messages from the Romney campaign, which is less focused on small-dollar donations.

Many of the Obama e-mails are close variations of each other, illustrating the sophisticated techniques used by campaigns to target different audiences. One recent message from Beyoncé was aimed at black women, while other pitches target Latinos, military families, pet owners and a host of other niches.

On May 31 alone, the ProPublica trove showed that the campaign sent at least 11 different fundraising pitches to different segments of its audience, including a general plea from Obama, another from Vice President Biden and a third version offering a free car magnet in exchange for a donation.

The messages tend to fall into two broad categories. First, there are the overly familiar, seemingly personal messages that are billed as coming from “Barack” or “Michelle” or “Joe” and are often pegged to a birthday, a dinner raffle or some other gimmick. The campaign issued a flurry of fundraising pleas pegged to Obama’s 51st birthday, which was Saturday, including a contest to attend a special event in Chicago later this month.

One strategist calls them the “stalker pitch.”

“I don’t get to tell you this enough,” starts one signed by the president.

“Me again,” writes Anne Marie Habershaw, the campaign’s chief operating officer.

“Hey,” starts another from Obama.

Then there is the “Chicken Little” approach — deployed with increasing frequency in recent weeks — in which Obama and other Democrats warn of sky-is-falling outcomes if they fail to raise enough money. “I’m going to be blunt,” Biden warned in one e-mail last week. “The campaign needs your support more than ever. We’ve been outraised by Mitt Romney and the Republicans — by tens of millions of dollars — for two straight months.”

Michael K. Wilkinson, a photographer from Washington who receives multiple Democratic fundraising e-mails each day, worries that the urgent messages from Obama are “making him look like he’s scared” and turning people off.

“The tone of the e-mails has shifted from ‘Hey buddy’ to ‘the ship is on fire,’ ” Wilkinson said. “Their whole strategy is based on grass-roots, small donors, which is great. But I worry that maybe they’ve over-relied on us to the point that not only are we not responding, they’re getting on our nerves.”

The Obama campaign, which declined to discuss its fundraising strategy in any detail, professed confidence in their approach. “We always take great care with those who wish to engage with us in any medium, including e-mails,” said spokeswoman Katie Hogan.

Many campaign strategists on both sides say e-mail fundraising has become so sophisticated that there is little chance the Obama campaign is angering too many followers. Peter Daou, a digital media strategist who worked for the presidential campaigns of Sen. John F. Kerry (D-Mass.) and then-senator Hillary Rodham Clinton (D-N.Y.), said electronic solicitation efforts “are almost a hard science at this point.”

“We had it down to such a science that we would study the position of the donate button, the shape and color of the donate button, the language used on a donate button, to determine what worked best,” Daou said. “Nothing in an e-mail solicitation from a campaign at this point is accidental. Nothing.”

The Romney campaign takes a more sedate approach in its fundraising e-mails, both in frequency and tenor. “Mitt just gets it,” reads a typical testimonial sent to donors last week from a Nevada candy company owner. “He gets how much hard work and perseverance it takes to make a go of it as an entrepreneur. And he understands that at the end of the day it’s people — not the government — who build successful businesses.”

A Romney campaign official, who agreed to discuss strategy in exchange for anonymity, said the campaign is consciously pursuing a lower-key approach and suggested the Obama campaign is sowing a sense of urgency to drum up funds.

“Obama is stuck with the problem of trying to re-engage people who were highly motivated for him last time,” said Jeff Roe of Axiom Strategies, which helps Republican congressional candidates. “If there was more excitement he wouldn’t have to be so aggressive.”

Of course, if donors are really turned off they can simply ask to be removed from a campaign’s e-mail list. Even then, the campaigns generally offer users a choice of receiving fewer communications, rather than none at all.

That’s the option chosen by Thomason, the Alabama Democrat, when she hit the “unsubscribe” buttons on e-mails from Brown and Obama. So far, she said, the move has significantly eased the volume of fundraising pleas hitting her inbox.

“Now that I get fewer e-mails it makes me pay closer attention to the ones they do send,” said Thomason, 48. “Maybe I’ll be more likely to open them.”