House Financial Services Chairman Barney Frank (D-Mass.) has some advice for liberal activists upset at the votes of the party’s conservative members: defeat them in primaries — but only in safe Democratic districts.

Frank told liberals at a Congressional Progressive Caucus foundation event in Los Angeles in April that primaries are the place to push for more liberal Members.

“I said don’t defeat conservative Democrats in November … the place to do that is in the primary,” Frank said Friday in an interview. But Frank added his caveat that such a primary challenge should only come in districts where a more liberal candidate would win in November.

“I particularly would not, for example, support a primary challenge against [Rep.] John Barrow [D-Ga.], because I thought that would cost us the seat,” Frank said.

Frank, who is a top party fundraiser, noted his support in primaries for other conservative Blue Dog Democrats besides Barrow, including Reps. Jim Matheson (Utah) and Allen Boyd (Fla.), for the same reason.

Frank’s comments come with the party’s liberal and moderate wings clashing both on and off Capitol Hill heading into what appears to be the most challenging environment for Democrats since the 1994 Republican revolution.

Frank’s support for taking on conservative Democrats in primaries was cited positively by the liberal blog Down With Tyranny in discussing controversial comments made last week by Energy and Commerce Chairman Henry Waxman (D-Calif.) to the Hill in a story headlined “Waxman sees bright side to November: ‘Difficult’ Democrats won’t be back.”

“I think a lot of the House seats we’re going to lose are those who have been the toughest for the Democrats to pull into line — the Democrats that have been the most difficult,” Waxman said.

While Waxman’s comments earned rave reviews from some left-wing bloggers, Blue Dogs fumed. They sought a rebuke from Speaker Nancy Pelosi (D-Calif.) and wanted a public apology from Waxman.

Waxman instead wrote a letter to the editor objecting to how the Hill represented the interview and vowing his support for all incumbents.

“For those who support universal health coverage, clean-energy policies, and economic stimulus for our struggling economy — as I do — there can be no bright side to the defeat of any Democratic members of Congress,” Waxman said in the letter.

Pelosi, in an apparent effort to quell the unrest, reiterated to the Democratic Caucus during this week’s special session her support for bringing back every Democrat in November.

“The Speaker’s highest priority is to re-elect every single incumbent Democratic Member of Congress, who have worked to create jobs, restore fiscal discipline and move America forward, not back,” said her spokesman, Nadeam Elshami. “The Speaker has said on numerous occasions: we will not yield one grain of sand.”

However, senior aides for moderate Democrats told Roll Call that Waxman needs to do more to repair his relationships with Blue Dogs, which they say were already frayed by Waxman’s handling of climate change and health care bills in his committee.

“He needs to pony up,” said one aide who complained that Waxman has been stingy with helping Democrats who opposed the health care and climate change bills. “He needs to demonstrate with his war chest that he wants these Members back.”

“I’d like to line them up and ask him which ones are too ‘difficult’ to come back,” fumed a former senior staffer for moderate Democrats. “Those are the types of things that make it tougher and tougher and tougher for the Democratic Caucus to work together collegially.”

The Waxman kerfuffle is emblematic of the broader fight between the two wings of the Democratic Party: Moderates feel leaders like Waxman and Pelosi walked them off a plank for issues like the climate change bill that went nowhere in the Senate, while liberals have been frustrated that moderates have slowed or watered down a host of agenda items.

Frank, for his part, said he disagreed with the notion that there is a bright side to losing Democratic seats.

“The goal should be the best possible Member elected in November from our standpoint,” Frank said. “I can’t think of any cases these days where that means you would replace a Democrat with a Republican.” Frank said that was not always the case, noting that former Rep. Chris Shays (R-Conn.) sometimes was more liberal than his opponent.

Getting rid of conservative Democrats via primary, however, is just fine, if it doesn’t threaten to flip the seat to a Republican, Frank suggested.

“I can think of a number of cases where a primary challenge would make sense,” Frank said, pointing to examples from the civil rights era and more recent Democratic primaries, such as Rep. Donna Edwards’ (Md.) defeat of incumbent Al Wynn in 2008 and former Rep. Hilda Solis’ (Calif.) victory over incumbent Matthew Martínez in 2000, both in solidly Democratic districts. “That’s the model.”

Frank did not name any particular Members that he thought should be challenged this year but said he cautioned liberals to think about what they are doing before they act.

“Don’t do things because they make you feel good,” he said.

Frank said that when liberals are frustrated with a conservative Democrat, they should “vote for that candidate in November but stick voodoo pins in him … that relieves your frustration and helps public policy.”