Nothing rallies US grassroots donors quite like a political affront, and Democrats facing a wave of Republican threats are cashing in on the negativity at the height of the 2014 election cycle.

Democratic campaign organizers in Washington have seized on major political assaults by Republicans -- including a demand for President Barack Obama's impeachment by Tea Party heroine Sarah Palin -- to sweep millions of dollars into their coffers and sign up thousands of new likely voters.

Despite Obama's approval rating being in the cellar, and the odds of Republicans taking back the Senate in an off-year election reaching 50-50, the Democratic Party has been the beneficiary of a flood of campaign funds not seen since Republicans were blamed for shutting down the government in October.

"Republican overreach continues to be a huge motivating factor for our grassroots," Democratic Congressional Campaign Committee (DCCC) spokesman Josh Schwerin told AFP.

The efforts kicked into high gear shortly after House Speaker John Boehner announced late last month that he was filing a lawsuit against Obama for abusing his executive authority.

Days later, a controversial decision by the US Supreme Court allowed employers on religious grounds to opt out of the health care law's mandate to provide contraception coverage.

Since Boehner's announcement, the DCCC alone has raised over $3 million online, from 157,000 donors, nearly a quarter of them first-time givers.

Their best fundraising day of the year came June 30, the day of the Supreme Court's 5-4 decision, when over $800,000 poured in.

Other Democratic groups cashed in too, including the Democratic Senatorial Campaign Committee.

"It's disgusting: The Supreme Court just ruled that corporations can deny women insurance coverage for birth control," the DSCC said in an email appeal strategically sent out about two hours after the decision.

The verdict could be good news for Democrats as they court single female voters. Most young women vote Democratic, and polls show a majority of young women care deeply about contraception coverage.

Then there is the issue many mainstream Republicans are wincing over: Palin's call for Obama's impeachment.

Boehner quickly said he would "disagree" with an effort to impeach the president.

But Democrats see money in Palin's announcement. About $400,000 has been raised by the DCCC since Tuesday, when the 2008 Republican vice presidential nominee called for Obama to go.

"First Boehner's lawsuit and now Palin's call for impeachment have led to an avalanche of support from low-dollar donors across the country," Schwerin said.

- 'Fundraising gold' -

Democrats are not blazing a new trail.

It is an age-old strategy that works equally well for Republicans, who raised millions off of the White House's bungled response to the anti-US attacks in Benghazi, Libya, or the investigation of Internal Revenue Service abuse.

And Republicans are fundraising on the very issues that Democrats are presently highlighting.

"Help us sue President Obama," blared one Republican National Committee fundraising email from July 2.

But Congressman Steve Israel, who heads the House Democrats' campaign arm, warned that Republican political agitation could be stirring up a hornet's nest ahead of November congressional elections.

"It's actually having the effect of exciting the Democratic base and independent voters who just don't want the Republican Congress to be suing another branch of the government," Israel told MSNBC television.

The DCCC this cycle has outraised its Republican counterpart $46 million to $17 million in low-dollar donations.

Bill Allison of the Sunlight Foundation, which helps track campaign spending and donations, said it was not unusual to see a party's coffers swell when its president is under assault.

"Sometimes a political setback in Washington can be fundraising gold," he said.