Former vice president Joe Biden has risen significantly in Democratic primary polls since he formally entered the race last month. While he is now clearly the front-runner, there are many reasons to think the race will tighten considerably by the end of the year.

Right now, Biden is about as close to a generic Democrat as you can get. He benefits from his long track record of supporting Democratic Party priorities and his eight years as President Barack Obama’s loyal deputy. Being out of office for about the past three years also means he has had the luxury to pick and choose his moments of public engagement, which allowed him to avoid many of the intraparty fights and controversies.

That’s going to change once he gets on a debate stage with his opponents. Progressives such as Sens. Bernie Sanders (I-Vt.) and Elizabeth Warren (Mass.) will presumably push him from the left, while others such as former representative Beto O’Rourke (Tex.) will likely make his age a subtle issue by proclaiming a need for new leadership. Biden will have to withstand that pressure, showing he is progressive and energetic enough to give his supporters confidence.