ALBANY — At first, he said he voted against expelling from the State Senate a lawmaker convicted in a domestic abuse case.

As it turned out, Sen. Antoine M. Thompson did vote to expel Hiram Monserrate, a Queens Democrat.

And now it seems Thompson opened up his campaign chest to donate to the defense fund to help Monserrate with his legal problems. Republicans are wondering if the donation was legal.

Looking for an explanation?

Thompson has been a difficult man to track down this past week.

The Buffalo Democrat was given an official “excuse” by the Senate to not attend session on Wednesday. That in of itself was an interesting move, because Democrats, with the loss of Monserrate, only have 31 votes to the Republicans’ 30 votes.

Thompson also was given a bye for Thursday, but then session was canceled because of a looming snowstorm.

Asked about his whereabouts Wednesday, a spokeswoman responded: “He’s out of town on business.”

Where? “I don’t know the exact itinerary,” Heather Zeist said.

“I just know he’s out of town,” she said, adding that he was in meetings “enlisting businesses to come to Buffalo.”

Requests to talk to Thompson went unanswered.

On Thursday, it was the same.

“Basically, all I know is what I told you yesterday,” Zeist said.

By Friday, calls to Thompson’s staff no longer were being returned.

Other senators and senior Senate officials did not know Thompson’s location or why he left town after Tuesday’s session.

One source said Thompson was on a “trade mission” but wasn’t certain if that was in the United States or overseas.

At issue is a $1,000 donation Thompson’s campaign sent last August and that was filed last month to the state Board of Elections. The recipient was listed: “HON LEGAL DEFENSE FUND.”

The purpose of the money transfer from Supporters of Antoine Thompson campaign committee was listed only as “Donations.” It was among the $87,000 Thompson’s campaign spent since last July on various expenses, including a New Year’s Eve party, restaurant tabs, printing, airfare, donations to other politicians — including Buffalo Mayor Byron W. Brown— and more than $5,000 in “unitemized expenses.”

The $1,000 August check was sent to the Queens home of P. Wayne Mahlke, a staff member of Monserrate. A legal defense fund was created last year to help Monserrate pay what was expected to be a tab well into the hundreds of thousands of dollars for his defense against charges he slashed his girlfriend in the face with a broken glass during an argument. Monserrate was cleared earlier this year on felony charges but was convicted of a misdemeanor abuse charge.

The apparent Thompson donation is curious, since he voted to expel Monserrate.

Right after the vote, Thompson told The Buffalo News he voted against the expulsion. He said the ouster was premature because Monserrate was still appealing his criminal case. The next day he called The News to say he was “tired and sick” when he commented the previous night on the Senate floor and that yes, as the record showed, he did vote to expel.

But Republicans say the donation could be more than curious. It is illegal for a legislative staff member to be the trustee of a lawmaker’s legal defense fund. In this case, Monserrate is not supposed to be informed who is giving him money, and the proceeds can be used only for legal expenses.

But Thompson sent his check to Mahlke’s home. And Mahlke said last week that he is not the fund’s trustee.

“I just passed along the donations,” he said.

To whom?

“I don’t reveal the name of the trustee. That’s a private matter,” Mahlke said, referring questions to Monserrate’s lawyer.

The lawyer, Joseph Tacopina, did not return calls for comment over a three-day period.

Republicans criticized Thompson, who has stepped up his public role as a Democratic voice against GOP senators in advance of the fall campaigns.

“When Senator Thompson speaks about ethics, campaign finance reform and the need for greater transparency and accountability in Albany, he apparently believes that none of that should apply to himself and his Democratic colleagues,” said John McArdle, a spokesman for Senate Minority Leader Dean Skelos, a Long Island Republican.

“He needs to come clean about his secret, and perhaps illegal, contribution to a legal defense fund that looks suspiciously like one set up for his disgraced colleague, Hiram Monserrate,” McArdle said. “Perhaps that’s why he said he didn’t vote to expel him after he did.”

The colleagues to donate to Monserrate’s fund are far from transparent and contributions are submitted to the state elections board in such ways as to make searching on the board’s Internet site challenging.

Senate Majority Leader Pedro Espada, a Bronx Democrat who voted against the expulsion, gave $2,000 in September to “HMLDF,” and the notation on the senator’s campaign filing listed no address; it appears that it could stand for Hiram Monserrate Legal Defense Fund.”

Sen. Martin Dilan, a Brooklyn Democrat who was also one of eight senators to vote against expulsion, gave $1,500 to something in Queens called the “HM Leagal (sic) Defense Fund.”