HARTFORD, Conn. — Pro-Israel groups, afraid of losing one of their staunchest supporters in Congress, are pouring money into beleaguered Senator Lieberman’s campaign as he tries to fend off a tougher-than-expected primary challenge.

The three-term lawmaker is struggling to dispatch millionaire businessman Ned Lamont in the August 8 primary. Mr. Lamont’s fierce criticism of Mr. Lieberman’s backing of the Iraq war and perceived closeness to President Bush has won him followers among hard-core Democrats.

Mr. Lieberman has said he will run as an independent if he loses the primary.

Pro-Israel political action committees have donated to the Connecticut senator’s campaign and have urged their national membership to give generously now and later, if Mr. Lieberman is forced to run as an unaffiliated candidate.

“Joe Lieberman, without exception, no conditions … is the no. 1 pro- Israel advocate and leader in Congress,” the chairman of the National Action Committee, a pro-Israel political action committee, Mark Vogel, said. “There is nobody who does more on behalf of Israel than Joe Lieberman. That is why he is incredibly important to the pro-Israel community.”

An Orthodox Jew who has never campaigned on Saturday, the Sabbath, because of his religious beliefs, Mr. Lieberman has been consistent in his support for Israel during his 17 years in the Senate.

In a recent interview with the Associated Press, Mr. Lieberman said Israel had a right to launch offensives in the Gaza Strip and Lebanon after two Israeli soldiers were kidnapped. He added that if Arab countries urged restraint by Hezbollah and Hamas, it “will allow the Israelis to cool down.”

Mr. Lamont, who is Episcopalian, has said America must “unambiguously stand with our ally,” but urged Mr. Bush to work on a peace settlement. He opposes negotiating with Hamas or Hezbollah.

Israel’s war with Hezbollah militants could result in an increase in donations to Mr. Lieberman, Mr.Vogel said.

“When there’s turmoil, people don’t like change,” he said.

According to the Center for Responsive Politics, Mr. Lieberman currently ranks fourth on the list of candidates receiving money from pro-Israel PACs in 2006. As of July 10, the most recent figures available, Mr. Lieberman had received $66,500 for the PACs out of $8.5 million raised.

Fourteen of the 32 PACs identified as pro-Israel by the center have contributed to Mr. Lieberman’s campaign, including $10,000 from the Women’s Alliance for Israel PAC, $5,000 from the Florida-based Sun PAC, and $2,000 from the New York-based Hudson Valley PAC.

Mr. Vogel’s group is reaching out to thousands of supporters in more than 35 states. For the first time, National Action Committee is inserting a separate flyer urging contributions to Mr. Lieberman in an invitation for an upcoming fund-raiser.

In 2000, Mr. Lieberman received $83,000 from pro-Israel PACs when he ran for his Senate seat, according to the center. He also was the Democratic vice presidential candidate later that year.

The founder of the pro-Israel Washington PAC, Morris Amitay, said increasing numbers around the country who support Mr. Lieberman’s pro-Israel stance are giving to his campaign because of the primary. Supporters also are volunteering to travel to Connecticut to help.

“It’s become such a big story and I think the polls have shown the race could be growing close and of course there’s concern,” Mr. Amitay said.

Mr. Lieberman’s campaign has been reaching out to pro-Israel supporters. Michael Kassen, a Westport, Conn., businessman who contributes to pro-Israel causes and various candidates, said the campaign has been in touch.

“This is a group that he can turn to,” said Mr. Kassen, who gave Mr. Lieberman $1,000 in 2005, according to campaign finance records.

Mr.Vogel said opinion within the pro-Israel community is mixed about whether America should have launched the Iraq war. Some prefer Mr. Lamont because they argue Iraq is more important than Israel, but Mr.Vogel said there is more agreement about staying in Iraq and getting the job done.

“We need to complete the war on terror, which does not include leaving because it sends the terrorists a very bad message: that terrorism pays,” he said. “Joe is part of that line of reasoning, which I think the pro-Israel community is pretty unified on.”