Rep. Elijah E. Cummings of Maryland, the ranking Democrat on the House Committee on Oversight and Government Reform, said in a statement that today is a "sad day for our country."
"We must investigate. And we must act on our findings," he wrote.
"@MerriamWebster definition of #collusion: secret agreement or cooperation especially for an illegal or deceitful purpose," Kennedy wrote.
Rep. Joaquin Castro, D-Texas, a member of the House Foreign Affairs and House Intelligence Committees, retweeted a post by Donald Trump Jr. and commented that the Russian government wanted to help the Trump campaign, there were Russian and American intermediaries involved and "the Trump family participated."
Trump supporters have a different view
Pro-Trump media figures, as has often been the case with controversial stories that have dogged the administration this year, put a more positive spin on the release of Donald Trump Jr.'s emails — and also attempted to use the release to criticize Clinton.
Charlie Kirk, the executive director for Turning Point USA, a conservative group, attempted to defend Trump based on personal experience.
Mike Cernovich, a pro-Trump journalist and blogger, suggested that both sides were funded by foreign powers.
"Ukraine and Saudi Arabia wanted Hillary to win, they funded and supported her," he wrote. "Russia was pro-Trump. Yet media only focuses on one side."
Jack Posobiec, another pro-Trump pundit, also tried to turn the conversation to Clinton.
"Don Jr releases his own emails and the media loses their minds. Hillary deleted 33,000 emails and the media tells us not to worry about it," he wrote.
And Gateway Pundit, a right-wing blog, appeared to find the emails exculpatory, writing that the "entire story is another big nothingburger."
But Rep. Lee Zeldin, R-N.Y., disagreed, saying that the emails released by Donald Trump Jr. show that the meeting "is a big no-no."
ABC News' Evan McMurry contributed to this report.