During the next few months, Sen. Patrick J. Leahy might be the most important Senate Democrat outside of Majority Leader Harry Reid.

Although the Vermont Democrat has been a fixture of the Senate for 38 years, the recently named president pro tem now finds himself at the center of two of the session’s most important debates and with one of the fullest legislative plates of any committee chairman since 2010.

Leahy has been tasked with shepherding immigration and gun control bills out of his committee in the coming weeks and months — both in regular order. For a chamber that spent the past two years in gridlock, it’s not an impossible task, but it won’t be easy either.

At a news conference last week, Reid was asked about the Judiciary Committee’s heavy workload and whether he had placed a priority on immigration over gun control or vice versa.

“We have one of the most senior members of the Senate who is chairman of that committee. He can do both. He can do both issues. And he’s going to do both issues. And we’re going to treat both issues the same way,” the Nevada Democrat said. “The Republicans — and rightfully so — feel better about bills that go through the committee structure. And we’re going to do that on immigration. We’re going to do it on violence, guns. Also, not only do Republicans feel better, so do we.”

Leahy, by virtue of his committee gavel, will largely determine which controversial issues get addressed in both measures. As the person who decides which version of both the gun violence and immigration bills to bring up, he will decide whether issues such as an assault weapons ban and a pathway to citizenship for undocumented immigrants get a leg up in the chairman’s mark.

When Sen. Daniel K. Inouye, D-Hawaii, died in December, Leahy passed on the opportunity take the helm of the powerful Appropriations Committee, deciding to stick with Judiciary instead. He made his announcement days after the shooting in Newtown, Conn., fully aware of President Barack Obama’s call to action on the issue.

An aide close to Leahy noted that the Judiciary Committee is one of the, if not the most, active committees in the Congress. “We have markups almost every single week, we have hearings every single week. The committee will be very busy, but he’s not afraid of a challenge,” the aide said.

As president pro tem, Leahy is now involved in more leadership meetings than ever before, and the success of immigration and gun control is not without leadership influence. Both the No. 2 and No. 3 Senate Democrats, Richard J. Durbin and Charles E. Schumer, serve on the Judiciary Committee. Both have also spoken out on gun control and are members of bipartisan group of eight Senators working on an immigration overhaul. Leahy also has worked at length with the No. 4 Senate Democrat, Patty Murray, on the pending Violence Against Women Act reauthorization, which leaders hope to approve this week.

Of the two biggest pending issues, however, gun control seems to be much more complicated. Reid long has had ties to the National Rifle Association, and it’s widely believed that stricter measures, such as the assault weapons ban championed by Judiciary member Sen. Dianne Feinstein, D-Calif., face long, if not impossible, odds for passage.

But the assault weapons ban is a perfect example of the political and policy balance Leahy must maintain in committee: Does he facilitate its inclusion to give it a leg up on the floor, but possibly imperiling the bill on the floor? Or does he try to force the amendment fight onto the floor, where the provision is almost certain to die?

Reid has said he will allow a floor vote on Feinstein’s assault weapons ban, but neither Reid nor Leahy appears especially committed to expending political capital to help it pass.

Moreover, Democratic leaders and Leahy have not laid out a path forward on gun control, opting to hold a series of hearings first before beginning their work on legislative language, which sources say the panel would like to finish by month’s end. That would set up the month of March for action on immigration.

“I don’t know. I wish I knew. It’d be so much easier if I did,” Leahy said on C-SPAN last month, when asked what could pass the Senate. “I don’t know. I would hope we could close the gun show loophole. I should hope we could limit the size of magazines being sold. I would hope that we would have background checks that are the same for everybody and actually have some teeth in them.”

Immigration might be an easier haul for the committee, as sources close to the bipartisan working group suggested they would provide the panel with legislative language upon which to base a markup, although historically this has not guaranteed a swift or bipartisan passage through committee.