WASHINGTON (Reuters) - Democratic presidential candidate John Kerry returned to Congress on Thursday for a series of party unity meetings and slammed a "Republican attack squad" that he said specializes in destroying foes.

Kerry, facing a flood of Republican criticism for characterizing his critics in the other party on Wednesday as a "crooked, lying group," refused to apologize and amplified his charges.

"There is a Republican attack squad that specializes in trying to destroy people and be negative," Kerry told reporters after having lunch in the Capitol with his Senate Democratic colleagues. "I think the president needs to talk about the real priorities of our country."

Kerry, preparing for his match-up with President Bush in November, also met with House Democrats and the black and Hispanic caucuses as he tried to line up support, hammer out a common message and begin to build a financial network for the campaign.

Kerry, who met on Wednesday with former rival Howard Dean, will meet later on Thursday with former rival John Edwards and about 100 of his top contributors.

Edwards, a North Carolina senator who dropped out of the race last week, planned the meeting to thank his top financial backers and invited Kerry to introduce himself and make a pitch for their support.

Kerry, a Massachusetts senator who has made only one visit back to the Capitol since the primary campaign started at the beginning of the year, was dogged by questions about his comments in Illinois.

Unaware he was being picked up by microphones, Kerry told a worker in a Chicago factory that his Republican critics were "the most crooked, you know, lying group I've ever seen."

REPUBLICANS ASK FOR APOLOGY

Republicans blasted Kerry and the Bush campaign asked him to apologize. Republican House of Representatives Speaker Dennis Hastert of Illinois said he took "great umbrage" at the remarks.

"He's getting off on the wrong foot in this campaign and name calling," Hastert said. "We're not lying when we say that Sen. Kerry is an old-fashioned Democrat -- tax and spend."

Kerry refused to back down, citing Bush's bitter presidential primary campaign against Arizona Sen. John McCain in 2000 and attacks on Georgia Sen. Max Cleland, a Vietnam veteran who lost three limbs in the war, in 2002 as examples of the "Republican attack squad" at work.

"I haven't said anything that's incorrect about them," Kerry said, explaining why he would not apologize. "They said lots of things that are incorrect."

Congressional Republicans welcomed Kerry back to the Capitol with a joint Senate and House leadership news conference to condemn his voting record on defense, energy, taxes and the economy.

"We're very glad to see Sen. Kerry back, the place hasn't been the same without him," joked Republican House Leader Tom DeLay of Texas, adding Kerry and Democrats had no agenda for the future and "haven't produced anything but hate."

"Listen to John Kerry, either off-microphone or on-microphone, it's all about how bad George Bush is and very little about what he would do and what is the agenda," DeLay said.

With Kerry's comments about his Republican critics "America got a little glimpse of the real John Kerry," he said.

New York Sen. Hillary Rodham Clinton said the Democratic meeting with Kerry was "upbeat and enthusiastic." She said Kerry asked for their help "and we're going to give him that help."

House Democratic Leader Nancy Pelosi of California said the meeting with House Democrats was "full of unity and commitment to each other to work together to win this election."

House members said Kerry did not discuss specific congressional agendas to help his campaign but explored the issues that will frame the national debate into November.