Boston — The tangled web of Boston politics is unraveling - with results that could seriously jeopardize the career of one of America's longest-serving mayors.
In recent weeks:
* The Boston Globe has reported that a federal grand jury, following leads from the state's Ethics Commission, is looking into a widespread ''cash laundering'' operation that tried to funnel up to $50,000 into four-term Mayor Kevin H. White's campaign chest.
* The US Department of Housing and Urban Development, charging misspending, has stripped $1.75 million from the city's $23 million annual community development block-grant. HUD officials are also probing charges that the city has mismanaged an additional $3.9 million in housing-program money.
* A federal grand jury has returned a 10-count indictment against the city's former budget director, William F. McNeill, charging him with fraudulently obtaining accident disability benefits from the city.
These latest developments come at the end of a year in which three other city employees have been convicted for bribery or extortion, and in which one of the mayor's political organizers had his disability pension revoked.
So far, no allegations have touched the mayor. But longtime observers see this as the strongest probe ever launched into the aura of corruption that many say has hovered around the White administration for years.
''He's been more illusive than the Scarlet Pimpernel,'' says one seasoned but apolitical observer, who sees these challenges as the most formidable the mayor has faced in his 15 years in office. Unlike previous attempts to investigate illegal operations within the mayor's political machinery, he says, this one is headed by ''a US district attorney who is really most serious about this.''
The Republican district attorney, William F. Weld, is accused by the Democratic mayor of pursuing a political vendetta. Mr. Weld takes a ''no comment'' approach, saying the matters are still under investigation.
The mayor, a charismatic and voluble Irishman often at his best when defending himself, has also lashed out at the Boston Globe. After supporting him for years, the paper wrote an editorial last July telling him it was ''time to leave gracefully'' rather than seek reelection in 1983.
The Globe, which last spring uncovered serious problems in the awarding of city retirement pensions, printed a story Dec. 17 suggesting the nature and extent of the current grand jury investigation. According to the story, a secret report by the Ethics Commission investigating a March 1981 birthday party planned for the mayor's wife, found ''evidence of possible cash laundering.''
Invitations to the party - which was canceled at the last minute - announced that guests could contribute to ''the Birthday Gift Committee.'' The Ethics Commission reportedly subpoenaed bank accounts from 75 of the 167 contributors - and in 64 cases found evidence that their contributions (many in the $500-$1,000 range) were simply pass-throughs of funds deposited in their accounts just before they wrote their checks for the party.
When the party was canceled, the mayor said all checks would be returned. Some accounts show that, after the returns, identical amounts of money were withdrawn - the inference being that the money was being passed back to its original source.
That original source may well be the major subject of grand jury investigation. Some observers and federal officials speculate that it may lie in funds illegally diverted from HUD grants. The commission reportedly found that one of the major fund-raisers was George Collatos, a former city employee now serving a federal prison term for extortion.
Many here say that the HUD recall and the retirement board issue - both of which could have arisen more from mismanagement than corruption - pale in comparison to the birthday-party investigation, which appears likely to grow increasingly far reaching.