Democrats have relished watching the Republican Party come under assault by a populist tea party insurgency. Now they may have to worry about their own revolt.

In some key U.S. Senate primaries Tuesday, Democrats are seeing liberals challenging party favorites by running the same kinds of populist campaigns harnessing anti-establishment energy as conservatives have.

Rep. Donna Edwards has mounted an unexpectedly competitive race for an open Senate seat in Maryland against party favorite Rep. Chris Van Hollen, whose long list of backers include Senate Democratic leader Harry Reid of Nevada.

In Pennsylvania, the populist campaign of former Representative Joe Sestak has triggered a big spending campaign by the Democratic Senatorial Campaign Committee to help push Katie McGinty, a former state official, as its preferred standard-bearer to unseat Republican Senator Pat Toomey in November.

Later in the year, liberal firebrand Alan Grayson will take on fellow Florida Democratic Rep. Patrick Murphy, who has the full backing of the Democratic Party, to be the nominee in the race for Marco Rubio's Senate seat. Combine that with the unexpected fervor surrounding Bernie Sanders' campaign for president and the Democrats could see an emboldened wing of populist, anti-establishment liberals in the coming years.

These heavily contested primaries hint at a real risk for the party, says Nathan Gonzales, editor and publisher of the Rothenberg and Gonzales Political Report. Party officials are finding it harder to help push the electorate toward the nominees they favor in Senate races, something Republicans have struggled with in recent cycles.

"For the last seven years, Democrats have enjoyed watching Republicans in disarray and unable to control their primaries," Gonzales said. "Now Democrats are going to have primary problems of their own. These primaries reflect a Democratic divide."

How the party's preferred candidates fare in these primaries could also help determine how much of a chance Democrats have to reclaim the Senate in the November election.

Gonzales said the new voters being fired up by Sanders' campaign have a tendency to think less of candidates who are picked by Democratic Party officials. It's something of a parallel with the tea party wing of the Republican Party, but instead of voters who think little of government, they "have a low view of the party establishment."

Neil Sroka, communications director for Democracy for America, one of several liberal groups backing Edwards and Grayson, says he does see a chance for outsider candidates this year, and said they are having an impact on the party's politics.

He pointed to Maryland's Van Hollen going from a willingness to consider a grand bargain in deficit talks to pushing to expand Social Security, and from being a backer of trade agreements to an opponent of the Trans-Pacific Partnership.

"Win or lose, these candidates are having an impact in a way that we haven't seen in some time, and they have a real chance of beating Washington's preferred candidates," Sroka said.

The Senate Democrats' campaign arm, the DSCC, hasn't endorsed in the Maryland race -- either Van Hollen or Edwards would be favored to keep the seat in deep blue Maryland in a presidential election year -- but there's no question most of the sentiment on Capitol Hill is in Van Hollen's favor.

The Maryland race especially has opened up fissures of ideological purity and race, with Edwards making explicit appeals for diversity in the Senate, where she would be the sole African-American woman.

Beyond that, Edwards has attacked Van Hollen's record as insufficiently pure in opposing cuts to entitlements like Medicare and Social Security.

And she's gone after Van Hollen for a compromise he crafted with the National Rifle Association on a campaign finance disclosure bill -- one critical to passing it out of the House. That attack has brought rebukes from Democratic leaders and irked Van Hollen.

But it's also the kind of issue Republican leaders have struggled with repeatedly with the ultra-conservative House Freedom Caucus -- is it better to push an ideologically pure agenda that fails to get the votes or make compromises for a broader goal?

Van Hollen has taken a line that he's the progressive with the long record of results, and said that's what Maryland progressives are looking for.

"They want results and tea party politics doesn't get you results," Van Hollen said in an interview after he greeted voters at the Greenbelt, Maryland, Metro station Thursday. "There's some things where we will fight to the finish -- a women's right to choose and protecting people's Social Security -- but obviously we should look for common ground where possible, because if you have people who are absolutely all or nothing, the people of Maryland get nothing."

Edwards said she finds Van Hollen's remarks "offensive" when he implies she isn't effective.

"All of us understand that in the legislative process it would be naive to think we can get everything perfectly," she said after a voter rally at New Shiloh Baptist Church in Baltimore Thursday evening. "The question is, where are you willing to draw that line?"

She pointed to her own backing of the Affordable Care Act as a sign of her willingness to compromise, but said some things should never be on the table. "It's one thing to negotiate and compromise, but we don't negotiate on principle, and for me Social Security isn't a program, it's a value."

She doesn't deny that Van Hollen is the favorite of many party leaders, but says that's not what the voters are looking for.

"What I've seen all across our state is that people are tired of a group of guys behind some doors picking and choosing our candidates," she said. "People are tired of that kind of status quo, establishment politics."

Sroka adds that there's a difference between the liberal groups' push for purity on issues like Social Security and the issues of the tea party.

"The difference is we're trying to encourage Democrats to embrace policies and issues and fights that are good on both policy and politics," he said.

Tea party positions would force a lot of Republicans to commit political suicide, he said.

"In this case, what we are saying is we want a candidate to be the next senator who knows not just which side she's on, but stands with us and is ready to fight," Sroka said.

Van Hollen, who gave up a House career that had him rocketing up the leadership ladder, appears to be surging in the polls over recent weeks as he's lined up a string of endorsements, including from The Washington Post and The Baltimore Sun.

But an Edwards victory, or near-victory, could help reshape the landscape when another third of the Senate is up for re-election in 2018.

--With assistance from Laura Litvan