Dorsey has come under increased pressure from conservatives, Republicans, and now the Department of Justice, who point to an apparent pattern of politically biased handling of users by the platform that has punished those on the right.
In a statement after the Senate hearing, the Justice Department said Attorney-General Jeff Sessions has "convened a meeting with a number of state attorneys-general this month to discuss a growing concern that these companies may be hurting competition and intentionally stifling the free exchange of ideas on their platforms". Many members of Congress fault social media platforms for failing to combat foreign efforts to influence US politics.
Daniel Castro of the Information Technology and Innovation Foundation said the intent of the statement was not clear but that the statement was worrisome.
Social media experts say trust is the cornerstone of their business and they must continue to evaluate the safety and security of their platforms. "In particular, law enforcement should not threaten social media companies with unfounded investigations for booting white nationalists like Richard Spencer off their platforms".
"When I think of our work, I think of my mom and dad in St. Louis, a Democrat and a Republican", Dorsey said.
Meanwhile, senators are debating new laws to regulate social-media companies.
Said Dorsey, "We do intend, and this will be an initiative this year, to create a transparency report that will make that data more public so that all can learn from it and we can be held public accountable".
Dorsey added that Twitter found itself "unprepared and ill-equipped for the immensity of the problems we've acknowledged", such as abuse, harassment, troll armies, propaganda, disinformation campaigns, and divisive filter bubbles.
Ms Sandberg admitted Facebook had been too slow to respond to manipulative messaging from Russian Federation during the 2016 election season, but had sharply raised its response and was more pro-active now.
Rep. Lynch beats Democratic primary challengers Wu, Voehl
Pressley also represents the latest underdog candidate to have emerged to challenge both establishment Democrats and Republicans. No Republicans were running, so only a write-in campaign in November could possibly stand between her and Washington.
"The committee takes this issue very seriously, and we appreciate that Facebook and Twitter are represented here this morning with an equivalent and appropriate measure of seriousness", Senator Burr of North Carolina said.
Trump, in an interview with The Daily Caller also on Wednesday, reiterated this perspective, saying that he believed tech firms "already" had interfered with the 2016 and 2018 elections.
But Mr Joe Barton, a Republican from Texas, told Mr Dorsey: "We wouldn't be having this discussion if there wasn't a general agreement that your company had discriminated against conservatives".
Lawmakers welcomed the comments but expressed concern about whether enough was being done.
Alphabet, Google's parent company, declined to send its chief executive Larry Page.
In a written statement, Google chief legal officer Kent Walker promised to maintain efforts to thwart foreign interference in USA elections.
The Senate hearing was the first of two on Capitol Hill focusing on social media, ahead of a House panel session where Dorsey was to speak on "transparency and accountability".
"Twitter can not rightly serve as a public square if it's constructed around the personal opinions of its makers", he told the panel.
Conspiracy theorist Alex Jones has also criticized the social media companies for banning him and his website Infowars from their platforms.
Trump added that "they are controlling what we can & cannot see".