WASHINGTON, July 11 — About 3 p.m. Tuesday, Senator John McCain ducked off the Senate floor, entered the Republican cloakroom and took out his mobile phone. Just hours after accepting the resignation of his two top campaign aides, he was making a conference call to his top fund-raisers to urge them to keep up the fight.

The call, however, may only have exacerbated an already tough week for Mr. McCain. Senate ethics rules expressly forbid lawmakers to engage in campaign activities inside Senate facilities. If Mr. McCain solicited campaign contributions on a call from government property, that would be a violation of federal criminal law as well.

There is no evidence that Mr. McCain has made a habit of making such calls or otherwise exploiting his office for political gain, and he is hardly the first lawmaker to call a donor from under the Capitol dome. But he made the call as he was in the spotlight because of the staff shake-up, sagging poll numbers and disappointing fund-raising of his Republican presidential primary campaign.

It was the kind of technical mistake that seasoned aides — like the ones his campaign is now letting go — are supposed to prevent.