The first step in any formal House Ethics Committee investigation would be a preliminary inquiry undertaken by Chairman Jo Bonner (R-Ala.) and ranking member Linda Sánchez (D-Calif.) to determine whether it is a matter serious enough to warrant the formation of an investigative subcommittee, which would then have the ability to discipline Weiner formally. Even if the inquiry does not progress to that stage, committee leaders could issue an informal report admonishing Weiner for his behavior, which could be politically damaging.
Herrington said even when an inquiry initially seems clear-cut, there is typically some "scope creep" once it begins.
"One of the things about an Ethics investigation is that once it starts, who knows where it leads," Herrington said.
The Ethics Committee might begin by getting computer records from Weiner's office and analyzing his email, Twitter and Facebook accounts, legal experts said. They would use that information to determine who will be called for questioning by the committee, either informally during a preliminary inquiry or subpoenaed during a formal investigation.
They can bring in "anybody that they think may have relevant information, and they can cast a wide net," Zuckerman Spaeder's Leslie Berger Kiernan said.
The legal fees incurred during a Congressional investigation can be steep.
So long as the scope of the inquiry relates to a Member's duties in office — and an Ethics inquiry into Weiner's communications would meet that criteria — the bills can be paid from money remaining in lawmakers' campaign accounts.
As of the end of the first quarter, Weiner had approximately $365,000 left in his campaign war chest, according to CQ MoneyLine. But legal bills can quickly amount to hundreds of thousands of dollars, and it could be a challenge for Weiner to raise more money in the future.
When Rep. Peter Visclosky (D-Ind.) was investigated in 2009 for campaign donations he received from members of the PMA Group, the legal fees paid from his campaign account jumped to nearly $250,000 from just $7,000 the year before, CQ MoneyLine data shows. The House Ethics Committee ultimately cleared Visclosky of any wrongdoing.
There would also be legal costs for any Weiner staffers who may testify in the matter, though in many cases those are also paid from Members' campaign accounts or a special legal defense fund.
"Legal expense trusts can be set up to benefit a Member, officer or employee of the House," Kiernan said.
The longer the scandal continues to unfold, the less likely it is to go away, experts agreed. Even if Weiner resigns, an inquiry into the circumstances surrounding the illicit communications could go forward.
"His resignation would obviously end his being subject to disciplinary action by the committee, but if there's some reason to believe there are institutional concerns, it could continue," said Robert Walker of Wiley Rein LLP.
Alex Knott contributed to this report.
This article updates the print version to include details on Democratic leaders’ response to the Weiner scandal.