Four days before the general election, Comptroller Alan Hevesi’s lead over his Republican opponent, J. Christopher Callaghan, has plummeted to the single digits, according to a new poll to be released today.

The sharp drop-off in support for Mr. Hevesi among statewide voters represents an extraordinary reversal of fortune for a candidate who was thought to be unbeatable as little as a month ago until he became ensnared in investigations into his use of a state employee as a chauffeur and personal servant for his ailing wife.

The poll, which was conducted by the Siena Research Institute, also found a high number of undecided voters, a further sign of decaying support for Mr. Hevesi, who is unloading millions of campaign dollars for a last-minute advertising blitz to salvage his reputation and political career.

Another poll released yesterday evening by NY1 cable news and Newsday showed a wider margin of support for Mr. Hevesi, with the comptroller ahead 48% to 38%.

Three weeks ago, Mr. Hevesi was sitting on a 27-point lead over Mr. Callaghan, a former treasurer of Saratoga County who has ridden a wave of bipartisan criticism of Mr. Hevesi to an improbable competitive position.

Since that time, the state ethics commission came out with a report that said there was probable cause to believe that Mr. Hevesi violated the public officers law by failing to pay the state back for his wife’s personal use of the employee, Nicholas Acquafredda.

The sudden implosion of Mr. Hevesi’s campaign has stirred hopes among Republicans that the party would avoid the first Democratic sweep of statewide offices in more than 60 years. Mr. Callaghan’s campaign got a surprise boost yesterday from a star of the national Republican Party, Senator McCain, who endorsed the candidate in a statement.

“Chris Callaghan has the integrity, the honesty and the decades of experience to restore faith in the Comptroller’s Office, and I know that he will make an outstanding fiscal watchdog for the people of New York State,” Mr. McCain wrote.

The Hevesi campaign, which has been trying to paint Mr. Callaghan as a backwater right-wing politician who lacks the experience necessary to run the $145 billion state pension fund, yesterday added to its list of Democratic endorsements the county executive of Nassau, Thomas Suozzi, who lost to Attorney General Eliot Spitzer in the Democratic primary for governor.

Even if Mr. Hevesi ekes out a victory on November 7, his political survival will be in doubt. Hanging over his head is a criminal investigation underway by the district attorney of Albany County, David Soares.

If Mr. Hevesi dodges an indictment and defeats Mr. Callaghan, he may be forced to step down by Mr. Spitzer, a former close ally of the comptroller.

A source close to the Spitzer campaign told The New York Sun yesterday that Mr. Spitzer would probably recommend that the state Senate remove Mr. Hevesi from office if the comptroller doesn’t resign before January. “He would likely make that referral,” the source said.

Mr. Spitzer last week withdrew his endorsement of Mr. Hevesi but said Mr. Callaghan wasn’t qualified for the job. The attorney general’s campaign has indicated that its preferred scenario would have Mr. Spitzer replace Mr. Hevesi with another comptroller nominated by Mr. Spitzer and approved by a vote of the full legislative body.

Meanwhile, Governor Pataki has appointed a former federal prosecutor, David Kelley, an attorney at Cahill Gordon & Reindel, to review the state ethics commission’s investigation and advise the governor on what punitive actions, if any, he should take against Mr. Hevesi. Mr. Kelley is expected to complete his report soon.

Complicating matters for Mr. Pataki and Mr. Spitzer are a number of knotty legal issues. If Mr. Pataki directs the Senate to put Mr. Hevesi on trial and the Senate votes to remove the comptroller from office before January 1, Mr. Hevesi would not be barred from retaking office when a new term begins. Mr. Pataki may be deterred from calling a trial because of its limited consequences.

Albany observers say it would be to Mr. Pataki’s advantage to pass the buck to the next governor than to take responsibility for a Senate trial that could produce its own assortment of legal complications. For one, lawmakers and the Pataki administration say the process for carrying out such a trial, which isn’t laid out in law, is a mystery.

It also isn’t clear if Mr. Spitzer would have the legal power to launch a removal proceeding to punish Mr. Hevesi for offenses that occurred during a previous term. There’s a question of whether a new Senate with different members has jurisdiction.

Depending upon the actions of the district attorney, such legal questions could become moot. No Senate vote would be necessary if Mr. Hevesi is forced to resign because of the criminal investigation.

If Mr. Soares concludes that Mr. Hevesi committed a crime, the comptroller may choose to accept a plea bargain to avoid a possible prison sentence. There’s speculation in Albany that Mr. Hevesi could follow in the footsteps of Brooklyn assemblyman Roger Green who pled guilty to filing false travel vouchers in 2004. He resigned from office months before the guilty plea and has since been re-elected to the Assembly.

In any event, the Hevesi situation could prove a distraction for Mr. Spitzer, if he is elected, during the first 100 days of his administration.