WASHINGTON — In a major economic policy speech today, Senator McCain will pledge to fix what he calls an “incomprehensible” and “Byzantine” federal tax code, casting himself as the candidate who will fight for changes that others have failed to achieve.

The speech to the Economic Club of Memphis is the second in a series of substantive addresses Mr. McCain is delivering in an effort to revive an ailing campaign and recapture the sharp-tongued candor that won him support in his first presidential bid eight years ago.

“It won’t be easy to fix a Byzantine code that has been decades in the making. But I don’t want the office for the sake of the nice house, the big plane, and the car and driver,” Mr. McCain plans to say, according to an excerpt of his remarks provided to The New York Sun. “I want to fix the hardest problems, and I’ll fight to make the tax code simpler, fairer, flatter, more pro-growth and pro-jobs.”

The Arizona senator is also planning to step up his attacks on Democrats for their handling of Iraq war funding legislation, this time calling them out for attaching unrelated “pork barrel” spending to the bill. In a speech on Iraq last week, Mr. McCain criticized Democrats for playing politics with funding for the war.

In Memphis today, he plans to accuse party leaders of trying to buy the votes of lawmakers who were undecided about the war. “To win their votes, Democratic leaders didn’t persuade them of the merits of their proposal. They bought them,” Mr. McCain will say. “They took the lid off the pork barrel and said to wavering members, ‘Help yourself, there’s plenty more where that came from.'”

An aide to the senator said he also will voice his support for entitlement reform, a line item veto for the president, and balancing the budget. Mr. McCain will deliver another speech on domestic policy next week ahead of his “official” candidacy announcement tour.

Mr. McCain’s speech comes as one of his Republican opponents, Mitt Romney, is calling on Congress to revert to a system whereby a three-fifths majority, instead of simply 51%, is needed to raise taxes. Mr. Romney, a former Massachusetts governor, promoted the idea in a speech in New Hampshire last night. The Republican-controlled Congress instituted the three-fifths rule when the party took power in 1995, but Democrats reversed it in January.

Mr. Romney has expressed support for a “simpler” tax code, but he has said he would not push for a complete overhaul. “I’m probably not going to be recommending throwing out the code and starting over,” he told the Des Moines Register earlier this month. “We can all dream about a simpler, fairer, flatter system.”

He has focused instead on lowering taxes across the board, and his campaign often notes that he was the first Republican candidate to sign a pledge to oppose any tax hikes.