WASHINGTON – Senator McCain of Arizona delivered a caustic rebuke yesterday to Senator Obama of Illinois, accusing the freshman of caving into pressure from his Democratic colleagues to scuttle new lobbying controls for partisan reasons.

In a sarcastic letter that parodied the usual senatorial courtesy, Mr. McCain faulted Mr. Obama for joining a meeting last week to discuss a bipartisan response to calls for lobbying restrictions and later withdrawing from the process in order to pursue an alternative advanced by Democratic Party leaders.

“I would like to apologize to you for assuming that your private assurances to me regarding your desire to cooperate in our efforts to negotiate bipartisan lobbying reform were sincere,” Mr. McCain wrote in the letter, which was released by his office. “I understand how important the opportunity to lead your party’s efforts to exploit this issue must seem to a freshman Senator, and I hold no hard feelings over your earlier disingenuousness,” Mr. McCain wrote.

Mr. McCain’s harangue apparently was triggered by a letter from Mr. Obama last week dismissing the Arizona senator’s call for a bipartisan working group to craft a proposal on lobbying. “I know you have expressed an interest in creating a task force to further study and discuss these matters, but I and others in the Democratic caucus believe the more effective and timely course is to allow the committees of jurisdiction to roll up their sleeves and get to work on writing ethics and lobbying reform legislation that a majority of the Senate can support,” Mr. Obama wrote. He added that public proceedings before Senate committees were the most likely course to substantive changes.

Yesterday evening, the Illinois senator said he was “puzzled” by Mr. McCain’s reaction. “I confess that I have no idea what has prompted your response,” Mr. Obama wrote in a new letter to Mr. McCain. “The fact that you have now questioned my sincerity and my desire to put aside politics for the public interest is regrettable but does not in any way diminish my deep respect for you nor my willingness to find a bipartisan solution to this problem.”

A spokesman for Mr. Obama, Robert Gibbs, said the senator has no desire to provoke a legislative stalemate that Democrats can exploit at the polls. “That is not our goal. Our goal is to change the way Washington works,” Mr. Gibbs told The New York Sun.

The public dressing-down of Mr. Obama by Mr. McCain, one of the Senate’s leading reformers, was striking because the Illinois senator has faced glowing press coverage and little criticism since he made his national political debut during the Democratic National Convention in 2004. Mr. Obama has since been dubbed one of the party’s rising stars.

Yesterday’s unusual senatorial exchange came as Republican congressional leaders were struggling to craft a response to the ongoing criminal inves tigation of a GOP lobbyist, Jack Abramoff. He pleaded guilty last month to corruption charges and admitted to offering bribes to members of Congress. Several congressmen have been implicated in court filings,but as yet none has been charged with a crime. One member, Randy Cunningham, a Republican of California, resigned in November after pleading guilty to fraud and bribery charges in an unrelated probe.

Some Republican members of Congress have argued that controls such as a ban on member travel paid for by outside groups are an overreaction to the lobbying scandal. Other members of the GOP caucus insist that if the party does not act decisively it could be trounced at the polls.Meanwhile,references to the “culture of corruption” have already become a staple of Democratic campaign rhetoric.

Senate Minority Leader Reid has introduced an ethics reform package that has the backing of 41 Senate Democrats. Two of those, Senators Lieberman of Connecticut and Nelson of Florida, also are supporting similar legislation Mr. McCain has proposed as a bipartisan vehicle for reform.

Aides to Messrs. Lieberman and Nelson said yesterday they were unaware of any pressure from the Democratic leadership to jettison Mr. McCain’s plan.