Sen. Rand Paul is trying to block the nomination of President Obama’s surgeon general nominee, Dr. Vivek Murthy, over what he calls “serious concerns” about Murthy’s leadership in a group that promotes ObamaCare and gun control.

Paul, R-Ky., told Senate Majority Leader Harry Reid in a letter that he objects to Murthy’s nomination because he questions Murthy’s ability to impartially serve as “America’s Doctor.” The letter marks an escalation in a simmering fight over the nominee, for a post that is typically not so controversial.

“Historically, the Surgeon General of the United States has been a position with the purpose of educating Americans so that they may lead healthier lives, rather than advancing a political agenda,” Paul wrote. “Dr. Murthy has disqualified himself from being Surgeon General because of his intent to use that position to launch an attack on Americans’ right to own a firearm under the guise of a public health and safety campaign.”

Paul is placing what's known as a hold on the nomination, a courtesy afforded to senators allowing them to voice their opposition. But Reid could technically still move forward, and a Reid spokesman suggested the majority leader would not sit on Murthy's nomination simply over Paul's objections.

"This is just one of the many ways Republicans exploit routine matters to impose gridlock on the Senate," he said in a statement. New Senate rules approved by Democrats could also make it easier to push through nominations like Murthy's.

Paul was hardly the only conservative raising concerns about Murthy. The doctor is the co-founder and president of Doctors for America, a group of over 16,000 doctors and medical students which heavily lobbied for ObamaCare. The group has participated in ObamaCare rallies, filed an amicus brief in the health law’s Supreme Court case and has promoted the law at the Republican National Convention, among other things.

According to Paul, the organization also has close ties to Obama’s political organization, Organizing for America. Paul says he has questions that need to be addressed over who is funding groups like Murthy’s.

“It is clear that more transparency is needed from a group whose influence frequently impacts appointments in the Obama administration,” he said.

Paul also voiced concerns over Murthy’s position on gun violence, noting Doctors for America asserts that gun violence should be considered a public health epidemic on par with heart disease and has pushed for gun control legislation to be passed.

White House spokesman Eric Schultz defended the nominee as a “dynamic, entrepreneurial practitioner who has dedicated a lot of time, energy, and passion to health and wellness.”

He added: “As Surgeon General he will be a powerful messenger on these issues and each of the tenets of health: nutrition, activity, and resilience.”