Rep. Seth Moulton, a frequent critic of the old guard of Democratic leadership in Congress, is now taking on one of his party’s progressive stalwarts.

The 47-year-old is running to unseat 80-year-old Sen. Ed Markey, who has represented Massachusetts in Congress since before Moulton was born.

Markey, a longtime champion of progressive causes, said he is more energized than ever to fight back against the Trump administration.

But Moulton warns there’s a risk in letting the party’s most senior members lead that fight.

“The reality is that if we win the United States Senate, it’ll probably be by one vote. We’ve seen what’s happened with Mitch McConnell. We’ve seen what’s happened with Joe Biden,” he told CNN’s Manu Raju.