The only real incident of note at last year's 'tea party' rallies on tax day, April 15, came when a passionate protester tossed a box of tea bags over the White House fence.

This year, things are looking a little dicier.

In towns large and small on Thursday, which is tax day, tea partyers are planning to march against big government – and the rhetoric is getting heated as counterdemonstrators mobilize. Some agents provocateurs say they’ll crash the tea parties, and the threat of fisticuffs and worse is hanging in the air.

IN PICTURES: Tea Parties

Both sides have been fueled by the Internet's churning of partisan, even extreme, politics. One commenter, a tea-party supporter, recently warned of a looming "low-grade civil war.”

But evidence, political scientists say, shows that political conflict rarely erupts in the United States once protesters and counterprotesters are actually toe-to-toe. In fact, partisans meeting on the street – away from their keyboards, TVs, and radios – may have a moderating effect.

"Mainstream politics is sort of like listening to two lawyers arguing, while [political outsiders] sound more like two guys getting ready for a bar fight," says political scientist Charles Franklin at the University of Wisconsin in Madison. "If opposition rallies start showing up or infiltrating the tea party, you do run the risk of fisticuffs. But you might have the opposite effect: If we're face to face, I might not actually punch you in the nose."

Yet the tension is palpable.

Crashing the tea party

A group called Crash the Tea Party is recruiting activists from New Hampshire to California to infiltrate hundreds of planned tea-party events. It’s urging activists to hold up racist signs in order to paint the movement in a negative light.

Tea partyers say they'll bring video cameras to root out any imposters.

In North Carolina, a new state regulation bans flags and signs from being carried on poles. This means that tax protesters can wave objects only as big as the flags handed out at a Fourth of July parade. According to state officials, the rule was put into place last September to prevent ralliers from inadvertently injuring one another.

But others suspect that the measure is aimed at suppressing intentional violence. Raleigh’s News & Observer writes, "The ban comes as, elsewhere in the nation, demonstrations have become heated and raucous."

"It's sad our country has come to this," Laura Long, who applied for a permit for a tea-party rally on Thursday, told the N&O.

Reports from New Orleans that a couple attending the Southern Republican Leadership Conference in the city was beaten up for wearing Sarah Palin pins were wrong. But that didn't stop Internet commenters from sensing a looming battle.

"If i am attacked at a rally ... someone will end up in their grave.... ENOUGH IS ENOUGH!!!!!" writes a commenter at the conservative Gateway Pundit blog.

And the Sooner Tea Party group in Oklahoma made news this week when it announced it was getting help from legislators to start a state-sanctioned militia to protect residents from illegal federal encroachment and in anticipation of civil breakdown.

Then again, not all threats are rhetorical, as this street fight in Florida last year shows.

Liberal activists say tea partyers ratcheted up the threat when they began carrying guns to protests. The counterprotesters have also been motivated by the provocative signs and rude epithets allegedly hurled by some tea-party members.

On liberal blogs, the phrase "violent racist teabaggers" occurs time and again.

"I predict the ... teabaggers will defeat themselves, and take down the Repubs with them. Then again never underestimate the power of stupid people in large numbers," writes a commenter on the progressive Truthout site.

The impact of agents provocateurs

Ultimately, agents provocateurs seldom make an impact, says Mr. Franklin, who is co-founder of Pollster.com. He points to Rush Limbaugh's exhortations for conservatives to vote in Democratic primaries during the 2008 presidential election. Such votes were essentially a nonfactor.

"Ultimately, the way the progressives win against the tea party is more through mainstream political action and less through tricks at campaign rallies," he says.

At a tea-party rally Wednesday morning in Boston, featuring Ms. Palin, liberal activists waving pro-Obama posters were keen to provide counterweight to a tea-party movement that polls show is itself becoming more mainstream. But the passion never devolved to violence.

Scattered in small clumps throughout the crowd, tea-party opponents reported receiving mixed reactions from rally attendees, says the Monitor's Will Buchanan, who was on the scene. Obama supporter Annie Reed of Brookline, Mass., for one, said she was called at various points a "communist" and "America hater." Someone suggested that she move to Russia.

But for others, the rally was an opportunity for constructive political discourse. One man in the crowd engaged several tea partyers in a measured back-and-forth discussion about the definition of patriotism and what "real American" means.

As for displays of anger and rudeness, something the tea-party movement has been criticized for, liberals in Boston showed they can dish it out, too. Derogatory statements about Palin were in evidence on signs and T-shirts sported by the counterprotesters.

IN PICTURES: Tea Parties