WASHINGTON — A fight over taxes and spending between Mayor Giuliani and Mitt Romney is setting the stage for a Washington summit on Friday in which the top Republican presidential contenders will lay out their plans for fiscally conservative economic policy.

Speaking in New Hampshire yesterday, Mr. Romney assailed Mr. Giuliani for opposing a line-item veto and a repeal of the commuter tax during his time at City Hall. Mr. Giuliani’s campaign shot back that Mr. Romney “failed to pass a single tax cut” as governor of Massachusetts.

Both candidates, along with a former Tennessee senator, Fred Thompson, are addressing the Americans for Prosperity Foundation’s “Defending the American Dream” summit Friday in Washington.

The clash yesterday reflects the wide-open status of the Republican race as it heads into its final months and signals that Mr. Romney in particular may be escalating attacks on Mr. Giuliani, who maintains a lead in national polls and appears to have closed the gap with Mr. Romney in New Hampshire.

Mr. Romney took the offensive against Mr. Giuliani at a town hall meeting when he was asked to differentiate his economic positions from that of his rival. Mr. Romney answered by highlighting the former New York mayor’s successful legal challenge to President Clinton’s use of the line-item veto, which allows a president to reject specific parts of a spending bill, such as earmarks. The Supreme Court ultimately ruled the provision unconstitutional following a lawsuit initiated by Mr. Giuliani. Mr. Romney called Mr. Giuliani’s move “a serious mistake,” saying the former mayor was responsible for killing the line-item veto, the Associated Press reported. He touted his own use of the measure to veto “hundreds of items” in Massachusetts.

He also criticized Mr. Giuliani for opposing then-Governor Pataki’s push to repeal the city’s commuter tax in the 1990s. “Can you imagine what would have happened up here in New Hampshire if I, as governor of Massachusetts, said everybody who commutes to Massachusetts has got to pay an extra special tax as a commuter? It just seems absolutely wrong,” Mr. Romney said, according to the AP. The Giuliani campaign responded aggressively yesterday morning, immediately setting up a conference call with Paul Cellucci, another former Massachusetts governor who is supporting Mr. Giuliani. Mr. Cellucci said Mr. Romney’s attack “appears to be some desperation” amid polls that show Mr. Giuliani erasing Mr. Romney’s lead in New Hampshire. He said they amounted to “pretty weak arguments for a governor who, in four years, really had no tax cuts for the people of Massachusetts.”

Mr. Cellucci, who served in the Massachusetts statehouse from 1997 to 2001, noted that Mr. Giuliani does support amending the Constitution to allow a line-item veto. Regarding Mr. Giuliani’s backing the commuter tax, Mr. Cellucci said only that Mr. Giuliani “had other priorities,” pointing to the mayor’s efforts to cut taxes in nearly two dozen other areas.

“I think the record is clear,” Mr. Cellucci said. “Rudy Giuliani cut taxes — broad-based tax relief for the families of New York and for the businesses of New York. Mitt Romney did not cut taxes in Massachusetts.”

A spokeswoman for the Giuliani campaign, Katie Levinson, added that Mr. Romney had “failed to pass a single tax cut as governor.” “Mitt Romney can repackage himself as many times as he wants,” she said, “but his failing fiscal record speaks for itself.”

A spokesman for Mr. Romney, Kevin Madden, replied: “Governor Romney is ready and able to defend his record of low taxes and pro-growth policies. He doesn’t need a surrogate to do it for him.” Messrs. Giuliani and Romney have each been touting their plans to lower taxes this week ahead of Friday’s summit. More than 1,000 members of the conservative advocacy group Americans for Prosperity have gathered in the capital to push an end to wasteful government spending and “pork barrel earmarks,” a spokesman, Ed Frank, said. He declined to weigh in yesterday on the feud between Messrs. Giuliani and Romney, or the fiscal records of the Republican candidates. “We’re going to withhold judgment until we hear what they have to say,” Mr. Frank said.

Senator McCain of Arizona was the first candidate to address the summit last night, lambasting the entire Washington establishment for letting spending get out of control and damaging the economy. In a particularly scathing rebuke, he said government had forgotten the words of President Reagan, who told university students in Moscow that freedom was “the continuous revolution of the marketplace.”

“One of the last vestiges of real socialism in the world is right here in Washington, D.C.,” Mr. McCain said. A campaign aide said the senator was not referring to either political party but instead saying that “Ronald Reagan’s call to freedom desperately needs to be heard in Washington today.”