AUGUSTA — The Republican chairman of a committee that will pick the winner of a disputed state Senate race in Portland’s northern suburbs said Monday that the panel will thoroughly review 21 ballots from Long Island that showed up during the recount of the contest but weren’t recorded on Election Day.

Sen. Roger Katz, R-Augusta, also said that the panel expects to hear testimony from the Long Island town clerk, who oversaw the election and signed off on the Election Day count.

“Obviously it’s political, but the most important thing to Senate Republicans is that the right person who was duly elected gets seated and we maintain the integrity of this institution,” Katz said. “And we intend to do that.”

The 21 ballots are at the center of the contested Senate District 25 race between Republican Cathleen Manchester of Gray and Democrat Cathy Breen of Falmouth. Unofficial results on the night of Nov. 4 showed Breen beating Manchester by 32 votes, 10,930 to 10,898. Manchester challenged the results, and after a recount, Manchester appeared to be the winner by 11 votes, 10,927 to 10,916.

During the recount, officials discovered 21 extra ballots from Long Island that were cast for Manchester. On Election Day, the town clerk’s list of voters who entered the polls and a count of the ballots showed that 171 were cast. After the recount, there were 192.

In addition to Long Island, District 25 includes Falmouth, Cumberland, Yarmouth, Gray, Chebeague Island and parts of Westbrook.

Democrats have suggested that the discrepancy indicates there was election fraud. The attorney for the Maine Democratic Party refused to sign off on the results of the recount, but didn’t officially challenge the 21 uncounted Long Island ballots until later.

Katz said the Senate would provisionally seat Manchester when the Senate is sworn in Wednesday, despite Secretary of State Matt Dunlap’s position last week that Breen should be provisionally seated as the unofficial winner based on the Election Day ballot count.

Katz spoke after Democrats called for an investigation into why there were 21 more ballots appearedduring the recount than there were Long Island residents recorded on the voter list. On Monday, outgoing Senate President Justin Alfond, D-Portland, and Democratic Party Chairman Phil Bartlett held a news conference at the State House urging Senate Republicans to conduct a thorough review.

The panel to be chaired by Katz will recommend a winner to the full Senate and will have broad discretion to hear testimony from witnesses, review evidence or simply declare Manchester the winner. Incoming Senate President Mike Thibodeau of Winterport has the authority to appoint the remaining six members of the panel, which will total four Republicans and three Democrats.

“What’s at stake here is not just about who takes the Senate seat on Wednesday,” Alfond said. “It’s about making sure that Maine citizens have faith in the integrity of our elections.”

He added, “The most important question that needs to be answered is why were there 21 more ballots than voters and how did this happen. … We need and expect our Republican lawmakers to be a partner in this endeavor.”

Katz, the current assistant Republican leader and a political moderate, confirmed that he would be chairman of the committee. He also refuted suggestions from Democrats that the panel would conduct a speedy review Wednesday, when the 127th Legislature is sworn in.

“There are some concerns about what happened on Long Island and we want to find out what happened as well,” Katz said.

Katz said the panel intends to hear testimony from Long Island Town Clerk Brenda Singo, who was the chief election warden on Nov. 4. However, it’s unclear who helped her count the ballots on Election Day. According to Maine election law, the warden counts ballots alongside a clerk from a different party. Both are involved in the certification of the count and signing off on the results before the ballots are sealed in a metal box with a state-issued lock.

Singo has referred all questions about the count to Dunlap, the secretary of state, who oversaw the recount. Reached Monday, Singo said she could not remember the names of other clerks involved in the count, but would try to answer the question Tuesday.

There are 238 registered voters in the town of Long Island, including 84 Democrats, 70 Republicans, 78 unenrolled and six Green Independents.

The town was not originally the source of contention in the recount. Kate Knox, the attorney representing the Maine Democratic Party, only officially disputed 10 missing ballots from Cumberland and Westbrook in the recount, even though those were not enough to overcome Manchester’s 11-vote lead.

Knox said the 21 Long Island ballots struck her as “very odd.” She refused to sign off on the recount, she said, after Bill Logan, the attorney for the Maine Republican Party, declined to take a second look at the Long Island ballots.

Democratic Party officials have since said that the Long Island discrepancy didn’t come into full view until last week, when Democrats reviewed the list of voters who showed up on Election Day and found only 171 names.

It’s unclear whether a review will solve a mystery that has sparked partisan rhetoric. On Monday, Maine Republican Party Chairman Rick Bennett slammed Democrats for “dishonestly attacking the election process and, apparently, the Maine State Police, by falsely raising the specter of ‘fraud.’ ”

The state police maintain custody of ballots involved in a recount.

Bartlett, the Democratic Party chairrman, said Republicans were turning a “blind eye to what may be ballot tampering.”

Katz said he did not know who would be the other Republican members of the committee. Possible Democrats include Sen. Bill Diamond of Windham, a former secretary of state who conducted a review of ballot tampering in 1992 that led to criminal charges against two Democratic aides. Diamond likely would be joined by Sens. Dawn Hill of Cape Neddick and Stan Gerzofsky of Brunswick.