The Massachusetts Senate initiated an Ethics Committee investigation on Monday night into its now former president Sen. Stanley Rosenberg in a dramatic day of upheaval that saw Worcester Democrat Harriette Chandler installed as the body’s acting president pending the outcome of a probe of alleged sexual harassment and Senate interference by Bryon Heffner, Rosenberg’s husband.

The election of Chandler, Rosenberg’s top lieutenant, and the adoption of an order green-lighting the Ethics Committee investigation marked the culmination of a marathon day of closed-door talks between Democrats and Republicans.

Chandler emerged as the unanimous choice of Democrats to take over the Senate temporarily after Rosenberg announced in the morning that he wanted to take a “leave of absence” from his leadership duties to ensure a “fully independent and credible” investigation.

“Choices had to be made and today we’ve chosen to move on and to move forward,” Chandler said. “What’s most important right now is that we work towards a swift and resolute conclusion to this whole sad event.”

Senators also said they welcomed a possible criminal investigation into the allegations published last Thursday in the Boston Globe, which reported that three men who do business on Beacon Hill were allegedly groped by Hefner and a fourth was kissed against his will. Those men also claimed that Hefner boasted about influence he wielded over Senate policy decisions.

The Ethics Committee will use an independent investigator to look into, according to the Senate order, “the conduct of Senator Stanley C. Rosenberg and whether he violated the rules of the Senate.” The investigator is to have “full access” to the Rosenberg’s office and his staff, and the committee was authorized to compel witness testimony and the production of “books and papers and such other records” by summons.

“One of the mistakes that anyone can make with an investigation is to try to limit it in too many ways. This investigation needs to take as long as it takes and it needs to go where the facts take it,” said Senate minority leader Bruce Tarr.

Rosenberg said last Friday that he is confident an investigation will show that Hefner, who is planning to seek treatment for alcohol dependence, had no decision-making role in the Senate.

Before the senators met on Monday, speculation had been rife about Rosenberg’s ability to cling to power and whether several senators jockeying for poll position to succeed Rosenberg could muster the votes to replace him.

Sen. Cynthia Creem, one of the Senate’s three assistant majority leaders, said there was no discussion of a new permanent leader because Rosenberg was only taking a leave of absence from the post. “At the end of the investigation, he may be Senate president again,” Creem said.

Asked if she expected Rosenberg to return to the presidency, she said, “I don’t have a looking glass.”

Matt Murphy, Michael P. Norton, Katie Lannan, and Colin A. Young of the State House News Service authored this report.