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:
* NEVADA - Senate Democratic leader Harry Reid offers the country's biggest target for Republicans, who accuse him of neglecting Nevada while waging national battles on behalf of Obama's agenda. Even after a heavy advertising blitz through the last three months of 2009, Reid trails potential Republican opponents by double digits in opinion polls in a state where the economy has nosedived.
* ILLINOIS - Obama's former Senate seat, left vacant by the departure of short-term appointee Roland Burris, will be the focus of an expensive and brutal battle between Republican Congressman Mark Kirk and Democratic state Treasurer Alexi Giannoulias. Republicans already have painted Giannoulias as a corrupt insider, while Democrats call Kirk a flip-flopper.
* DELAWARE - Republican Congressman Mike Castle, one of the party's last House moderates and a popular fixture in state politics for more than 40 years, is a big favorite to win Vice President Joe Biden's old seat. Castle's candidacy was bolstered when Biden's son, Democratic state Attorney General Beau Biden, passed on the race.
* NORTH DAKOTA - The retirement of three-term Democratic incumbent Byron Dorgan put this Senate seat in play in a rural, conservative, Republican-leaning state. Democrats are still uncertain on their candidate, giving popular Republican Governor John Hoeven a clear path and a running head start.
* ARKANSAS - Democratic incumbent Blanche Lincoln trails all her potential Republican opponents in opinion polls in another conservative state where her vote to back Obama's sweeping healthcare overhaul was highly unpopular.
* PENNSYLVANIA - Democratic incumbent Arlen Specter, who switched parties last year because he feared he would lose as a Republican, might fare no better as a Democrat. He faces a strong Democratic primary challenge from Congressman Joe Sestak. If he survives, he will face former Congressman Pat Toomey -- who almost beat him in a Republican primary in 2004.
* COLORADO - Democrat Michael Bennet was the Denver public school superintendent when he was appointed to the Senate last year to succeed Ken Salazar, who became Interior secretary. Bennet has been successful raising money but faces a primary fight from a former Colorado House speaker. If he survives, he will have tough competition in November from the probable Republican candidate, former Lieutenant Governor Jane Norton.
* INDIANA - Evan Bayh, a two-term senator, former governor and son of former senator Birch Bayh, has been a strong political brand in Indiana for decades but could face a tough Republican challenge from former Senator Dan Coats. Since his name surfaced earlier this week, Coats has been hammered by Democrats for his work as a lobbyist and for living and voting in Virginia for the past decade.
* CALIFORNIA - Incumbent Barbara Boxer appears vulnerable and could face a strong Republican challenge from either former Hewlett-Packard executive Carly Fiorina or former Congressman Tom Campbell. But California remains a Democratic state with expensive media markets. National Republicans might decide to sink their money into more promising races.
* CONNECTICUT - With the retirement of unpopular incumbent Chris Dodd, Democrats improved their chances of holding this seat. It cleared the way for Richard Blumenthal, the popular Democratic state attorney general, who has a big early lead on his potential Republican opponents -- former Congressman Rob Simmons, former wrestling executive Linda McMahon and businessman Peter Schiff.
(Editing by Eric Walsh)