U.S. Rep. Barney Frank said Friday U.S. Rep. Mazie Hirono would be a vote for equality, tax fairness and environmental protection in the U.S. Senate, casting her election as important for Senate Democrats and President Barack Obama.

Frank, who serves with Hirono in the Congressional Progressive Caucus, the Democrats’ most liberal wing, appeared at a fundraiser for Hirono on Maui. In an interview, the Massachusetts Democrat described former U.S. Rep. Ed Case, Hirono’s primary opponent, and former Gov. Linda Lingle, the leading Republican contender, as potential obstacles for the Hawaii-born Obama if the president were to win re-election in November.

"One of the most important things about Mazie Hirono’s race is, ‘Does the Obama presidency get frustrated in the Congress?’ It’s partly with Ed Case but obviously even more with Linda Lingle," he said.

"If the Republicans have the House and the Senate, then Obama’s second term is going to be a very difficult one. But still better than a (Mitt) Romney first term."

Frank is supporting Hirono in the primary over Case, who aligned himself with the moderate-to-conservative Blue Dogs when he served in Congress. Frank said the Blue Dogs have held back progressive ideas and have been the least supportive of Obama.

"I understand the need for people who represent North Carolina, Arkansas. When those Democrats are Blue Dogs, I understand that," he said. "That’s the best we’re going to get."

Both Case and Lingle are campaigning as moderates who would work in bipartisan fashion with centrists in the Senate. But Frank said national Republicans have become a hard-line, right-wing operation and have marginalized moderates. He said the only way moderates would again have a voice in the GOP would be for the party’s right-wing experiment to fail.

"Unfortunately, the ability of a self-styled moderate Republican to have any impact has disappeared," he said.

Frank said if Republicans take control of the Senate, conservatives would lead key committees that oversee commerce and environmental protection and would be in a stronger position to thwart Obama. He said Lingle would be "part of a crew that will try very hard to wreck the Obama presidency."

Case said he likes and respects Frank, who has chosen not to run for re-election after serving in the House for 32 years. "But his and Mazie’s extreme-left fiscal and economic policies and endless partisan warfare are no more the solution for our country than the far right’s," the former congressman said in an email.

Case cited the House’s rejection in March of the federal budget proposed by the Congressional Progressive Caucus, which was voted down 346 to 78. He noted that 107 Democrats, including U.S. Rep. Colleen Hanabusa, D-Hawaii, voted against the liberal alternative.

"We need more senators with workable mainstream solutions to grow our economy and fix our country’s finances, and voters in Hawaii want that just as much as those of any other state," he said. "When will Mazie stop hiding behind everyone else, speak for herself, and debate these critical issues with me straight up across Hawaii?"

David Chang, chairman of the state GOP, called Frank a polarizing liberal whose lack of oversight on Fannie Mae and Freddie Mac contributed to the nation’s subprime mortgage crisis in 2008. Frank, as a leader on the House Financial Services Committee, had described the government-sponsored financial institutions as sound in the years before the crash.

"I don’t know if he’s the type of guy you would want here in Hawaii right now," he said.

Chang said Lingle, by comparison, had moderate U.S. Sen. Susan Collins, R-Maine, campaign for her in Hawaii.

Frank said a better example of the challenge for moderate Republicans would have been U.S. Sen. Olympia Snowe, R-Maine. Snowe announced her retirement after concluding that the partisan nature of the Senate would not soon change.

"I think a better indicator here of what’s happening is Sen. Snowe rather than Sen. Collins," he said. "She quit."