The latest round of subpoenas from the congressional committee investigating the Jan. 6 Capitol attack suggests the panel is deepening its probe of how allies of former President Donald Trump urged legislatures in key swing states to subvert the 2020 election.

But the subpoenas also shine a spotlight on a leading contender in Pennsylvania’s Republican primary for governor: State Sen. Doug Mastriano.

In a letter to Mastriano, the committee demanded documents and testimony regarding his involvement in a plan to send a pro-Trump slate of electors to Congress from Pennsylvania, despite Joe Biden’s victory; his conversations with Trump; and his presence on Capitol grounds the day of the riot.

Here’s what to know about Mastriano:

Who is Doug Mastriano?

Mastriano, 58, is a retired Army colonel from Franklin County, in south-central Pennsylvania. He’s a relative newcomer to politics, having been first elected to the state Senate in 2019. But since then, he’s become one of the highest-profile political figures in Pennsylvania.

He started to gain a following early in the pandemic, as he held rallies outside the statehouse protesting Gov. Tom Wolf’s coronavirus restrictions. And he grew a big audience online with what he called “fireside chats” on his Facebook page, urging his followers to “walk as free people.”

But it wasn’t until the 2020 election that he got national attention.

What is Mastriano’s stance on the 2020 election?

Mastriano is one of the leading election deniers in Pennsylvania. A day after state officials certified Biden’s victory in Pennsylvania, he organized a legislative meeting in Gettysburg, inviting the public to share allegations of voter fraud.

As in several other swing states Biden won, Trump lawyers Rudy Giuliani and Jenna Ellis showed up. Trump himself called in to the meeting from Washington, offering his input via speakerphone: “This election was rigged, and we can’t let that happen.”

» READ MORE: From 2020: Trump takes complaints over Pa. election before friendly state lawmakers in Gettysburg

Shortly after, Mastriano met Trump at the White House — and was pulled out of the West Wing when the senator was told he’d tested positive for the coronavirus, the Associated Press reported.

Three days later, on Nov. 28, Mastriano tweeted incorrectly that there was “mounting evidence” Pennsylvania’s election was compromised — in which case, the senator said, the state legislature “has the sole authority to direct the manner of selecting delegates to the Electoral College.”

That statement was consistent with a widely discredited legal argument developed by Trump allies claiming Vice President Mike Pence could reject pro-Biden slates from key swing states such as Pennsylvania.

By then, Mastriano was firmly on Trump’s radar. “Doug is the hero,” Trump said on Dec. 5, speaking to Mastriano while the senator was participating in a radio interview.

» READ MORE: From 2020: Fact-checking false claims about Pennsylvania’s presidential election by Trump and his allies

Around that time, Mastriano and another Pennsylvania Republican — U.S. Rep. Scott Perry — directly contacted Deputy Associate Attorney General Richard Donoghue, urging him to investigate allegations of voter fraud, according to a report later compiled by the Democratic-led U.S. Senate Judiciary Committee.

Trump, during a Dec. 27 phone call with Donoghue and acting Attorney General Jeffrey Rosen, mentioned Mastriano and Perry by name. “Just say the election was corrupt and leave the rest to me and the Republican congressmen,” Trump said on the call, according to the report, which was based on interviews with Justice Department officials.

Mastriano didn’t dispute the evidence in the Judiciary Committee report but said its allegations were “part of an ongoing, desperate attempt to distract from what progressive policies are doing to our country.”

He joined several of his Pennsylvania Senate GOP colleagues in signing a Jan. 4 letter urging Congress to delay certification of the Electoral College results.

Was Mastriano in Washington during the Capitol attack?

Mastriano’s campaign spent thousands of dollars on buses days before Jan. 6, and the senator posted on Facebook at the time offering bus rides to Washington that day. He has said he and his wife left the Capitol grounds before the riot ensued, but video footage appears to show them among a crowd passing through breached barricades set up by police to keep rioters at bay.

Mastriano has said he followed Capitol police directions and respected police lines as they “shifted.”

No footage has surfaced showing him inside the Capitol.

» READ MORE: From 20201: Mastriano says he left the Capitol area before the riot. New videos say otherwise.

Over the last year, Mastriano has continued to spread baseless claims of widespread fraud. In June, he traveled to Arizona to get a firsthand look at the widely discredited partisan election review commissioned by Republican lawmakers there.

Back in Pennsylvania, Mastriano sent letters to three counties, including Philadelphia, demanding voting equipment and virtually all election-related materials as he pursued what he called a “forensic investigation.”

The counties refused to comply, and Mastriano was later stripped of his committee chairmanship by Senate President Pro Tempore Jake Corman (R., Centre), who took control of the review and is now also running for governor.

Where does Mastriano fall in the race for governor?

Mastriano announced his campaign for governor last month at a rally in Gettysburg, where he was joined by Michael Flynn, the former Trump national security adviser and prominent election denier.

He has consistently won the support of about 20% of Republican voters in gubernatorial primary polls, often leading the field, according to GOP insiders who have seen internal data. A public poll released last week by Trafalgar Group showed Mastriano with 19.9%, trailing only former U.S. Rep. Lou Barletta. Corman, the next-closest candidate, got 5%. More than a dozen Republicans are running.

» READ MORE: Lou Barletta is the best-known GOP candidate for Pa. governor. Is that enough?

Mastriano reported having about $550,000 in his campaign account to start the year — more than Barletta but less than several other rivals who are already spending money on TV ads.

The primary election is May 17. State Attorney General Josh Shapiro is the only established Democrat running.

Is Mastriano’s subpoena a big deal?

The congressional committee demanded Mastriano turn over relevant materials by March 1 and appear for a deposition March 10.

The senator’s spokesperson didn’t respond to a request for comment Tuesday.

It remains to be seen whether he will comply. Congress has voted to hold two people in contempt for failing to comply with the Jan. 6 committee’s subpoenas, and the Justice Department has charged one of them, Steve Bannon, with a crime. Bannon pleaded not guilty.

Given the extent to which Republican voters have wrongly come to see the 2020 election as stolen, it’s possible that being subpoenaed will only improve Mastriano’s prospects in the primary.