Republican Scott Brown's formal arrival in the U.S. Senate on Thursday robbed Democrats of their crucial 60-vote supermajority and set the stage for a bitter November election fight for control of the 100-member chamber.

Public dissatisfaction with the economy, unemployment and President Barack Obama's agenda helped fuel Brown's election in heavily Democratic Massachusetts last month, sending shock waves through Democratic circles.

With big majorities in both the Senate and the House of Representatives, Democrats are still heavily favored to retain control of both chambers in November but face growing worries about the extent of their possible election losses.

To regain Senate control, Republicans must win 10 states held by Democrats while retaining all of their own seats, including several vulnerable ones. That will be difficult.

Here are the 10 most vulnerable Democratic Senate seats in November: