"I am going to run the most vicious, ruthless campaign to dethrone Tim Kaine," Stewart said on Facebook Live
outside his Virginia home. "We're not holding back any punches. I'm going to go after him very, very hard."
Stewart, who serves as the Prince William County Board of Supervisors chair, narrowly lost
the Republican nomination for governor to former Republican National Committee chairman Ed Gillespie by one percentage point last month.
His campaign website
, also launched Thursday, currently boasts more than 17,000 supporters.
Stewart cited the closeness of the gubernatorial race as his motivation for pursuing the Senate seat. He said the Republican Party has been fighting a "UFC fight" to improve the economy and added that he wants to "get back in the ring."
"There's an appetite for a Republican fighter. They're looking for a vicious, ruthless, Republican, conservative fighter," Stewart said.
Kaine, who was Democratic presidential nominee Hillary Clinton's running mate in 2016, is a first-term senator who previously served as the state's governor and the mayor of Richmond, and also was the chairman of the Democratic National Committee.
Stewart said he plans to support conservative Sens. Ted Cruz, Mike Lee, Ron Johnson and Rand Paul and call for a full repeal of the Affordable Care Act. Later, he likened himself to President Donald Trump.
"I've always said that I was Trump before Trump was Trump," Stewart said, adding: "I speak my mind, and sometimes it gets me in trouble."
Stewart, however, was fired
from his unpaid position as the Trump campaign's Virginia chairman in October after staging a controversial demonstration against the Republican National Committee for supposedly abandoning its candidate. The incident, however, has not eroded the Virginia Republican's support of the President.
In his official statement Thursday, Stewart referred to Kaine as "the leading obstructionist against Trump's America First agenda." He also critiqued the senator's focus on Russia in his live announcement, calling the investigation "a bunch of bollocks."
Kaine also made headlines this week for saying Donald Trump Jr. may have committed treason
by meeting with a Russian lawyer in order to get opposition research on the Democratic presidential candidate, Clinton.
"That, the investigation -- it, it's not -- nothing is proven yet. But, we're now beyond obstruction of justice in terms of what's being investigated," the Virginia Democrat told CNN Tuesday. "This is moving into perjury, false statements and even potentially treason." A Kaine spokesperson later sent a statement to CNN saying that the matter of treason will be up to investigators to prove.
In the past year, Stewart has garnered attention
for controversial behavior, including an appearance on a show
hosted by Mike Cernovich, a member of the pro-Trump, far-right media who has dismissed the existence of date rape, as well as a Reddit "Ask Me Anything
" session on the site's largest pro-Trump forum, where, among other things, he called Gillespie a "cuckservative."
This story has been updated.