Rep. Marsha Blackburn (R-Tenn.). (Marvin Joseph/The Washington Post)

Rep. Marsha Blackburn (R-Tenn.) is running for Senate in a state that has been hit hard by the opioid crisis. Now, she is fielding attacks for supporting a bill that weakened law enforcement’s ability to curb the flow of pain pills to the black market.

James Mackler, the Senate race’s Democratic front-runner, criticized Blackburn over her involvement in a bill that undercut the DEA as opioid deaths were on the rise in Tennessee.

“I’m running for U.S. Senate because Tennesseans need a senator that will stand up for them rather than catering to special interests and corporate lobbyists,” Mackler said in an emailed statement Sunday night.

“That Congresswoman Blackburn would champion legislation like this while Tennesseans face an opioid epidemic is all one needs to know about her priorities,” he said.

Blackburn and Mackler are the best-known candidates running to replace Sen. Bob Corker (R-Tenn.). As a Republican, Blackburn has an immediate advantage in the state, which hasn’t sent a Democrat to the Senate since 1990. But Mackler, an attorney and Iraq War veteran, could use an issue such as the opioid epidemic to peel off some of her support if they face each other in next year’s general election.

Blackburn co-sponsored the Ensuring Patient Access and Effective Drug Enforcement Act, which raised the DEA’s standard for freezing suspicious drug shipments. Her role in securing the bill’s passage, as well as the pharmaceutical industry’s contributions to her campaign, are detailed in a new Washington Post/“60 Minutes” investigation.

On Monday, Sen. Joe Manchin III (D-W.Va.) urged that President Trump withdraw the nomination of Rep. Tom Marino (R-Pa.) as drug czar in light of the probe’s findings.

Blackburn’s campaign did not respond to a request for comment Sunday night. Her office did not respond to Post investigators’ prior requests for an interview.

The conservative Republican and eight-term congresswoman announced her bid to replace the retiring Corker earlier this month. A surrogate for President Trump during the 2016 election, Blackburn is closely aligning herself with the White House in an effort to secure support from his base in the GOP primary.

“I’m politically incorrect and proud of it,” Blackburn said in a video announcing her campaign. “I believe in President Trump’s immigration ban, and I’ll fight with him every step of the way to build that wall.”

Trump beat Democrat Hillary Clinton by 26 points in Tennessee in November, although there are emerging signs of trouble for him in the state: A survey released last week by Morning Consult found that Trump’s approval rating there has fallen 23 points since he took office.

In a sign that conservative support is coalescing behind her campaign, Blackburn has already received endorsements from the Club for Growth and Citizens United, the influential conservative outside group run by former Trump deputy campaign manager David Bossie.

“We support Marsha Blackburn for Senate because she understands that you can’t just talk about a conservative agenda, you have to fight for it and get it enacted,” Bossie said in a statement Tuesday. “Marsha Blackburn is a full spectrum conservative who fought the Obama agenda every step of the way and fully supports President Trump’s agenda of repealing and replacing Obamacare, tax reform, and border security.”

Blackburn’s support from Citizens United drew criticism from the Tennessee Democratic Party, which has accused her of being in thrall to big business.

“It is no surprise that Rep. Blackburn would welcome the support of a dark money organization like Citizens United,” said Mary Mancini, chairwoman of the Tennessee Democratic Party, in a statement Wednesday. “She has been in Washington for 16 years doing the bidding of corporate special interests.”

Blackburn entered the race after Gov. Bill Haslam, who would have been a formidable primary opponent, decided he would not run.

She will face at least one other Republican — conservative activist Andy Ogles — in the primary. Another possible contender, former congressman Stephen Lee Fincher (R-Tenn.), is expected to announce this week whether he will enter the race.

Attempts to reach Ogles and Fincher were not successful.

Blackburn’s pitch to voters is centered on her conservative bona fides. In her announcement video, she slammed Senate Republicans for failing to pass a bill replacing Obamacare, calling it a “disgrace,” and she touted her winning fight as a state legislator to prevent the resurrection of an income tax in Tennessee.

In Congress, Blackburn is best known for leading a House investigation into Planned Parenthood and sponsoring legislation that dismantled Internet privacy rules this spring. She is one of House Republicans’ most senior and prominent women, making regular appearances on cable news shows.

Former speaker John A. Boehner (R-Ohio) named Blackburn the head of the House Select Investigative Panel on Infant Lives in 2015, tasking her with probing ties between abortion providers and medical researchers after conservative activists released secretly recorded videos they claimed revealed an illicit trade in fetal tissue.

Mired in partisanship from the beginning, the panel was dissolved in January. But Blackburn’s work as its leader received a new wave of attention this month after a controversy that could win her sympathy from conservatives.

In her announcement video, Blackburn claimed she “stopped the sale of baby body parts,” a comment that led Twitter to suspend the clip in promoted tweets for violating its advertising rules. A day later, after backlash from conservatives, the website reversed its decision.

Blackburn reportedly sought to raise funds from the flap.

“I’m being censored for telling the truth,” Blackburn wrote in a fundraising email, according to the Hill. “When I talked about our legislative accomplishments to stop the sale of baby body parts, they responded by calling our ad ‘inflammatory’ and ‘negative.’ ”

A longtime member of the House Energy and Commerce Committee, Blackburn now leads the panel’s communications and technology subcommittee.

Earlier this year, she sponsored a bill to kill landmark privacy regulations that would have banned Internet providers such as AT&T and Comcast from storing and selling customers’ browser histories without their consent. Trump signed the killing measure into law in April.

“[Consumer privacy] will be enhanced by removing the uncertainty and confusion these rules will create,” Blackburn said this spring.

With her trademark heavy glasses and blonde sweep of hair, Blackburn is known as a colorful character on Capitol Hill, adept at throwing red meat to the conservative base.

In 2014, she debated the existence of climate change with the scientist Bill Nye on NBC. During the 2016 Republican convention, she quoted blue-collar celebrity Larry the Cable Guy in her speech endorsing Trump. That fall, during an appearance on CNN, she denied the existence of institutional racism and endorsed the New York Police Department’s former “stop-and-frisk” policy as a tool for halting crime.

“I know the left calls me a wing nut or a knuckle-dragging conservative,” she said in her announcement video. “And you know what? I say that’s all right. Bring it on.”