Democrats appear all but certain to expand their 51-49 Senate majority this year. Republicans are defending 23 Senate seats, including five left open by GOP retirements, while Democrats have just 12 senators up for re-election and no departures. Here’s a look at the most competitive races.

Possible Democratic pickups:

. Alaska. Democratic Anchorage Mayor Mark Begich will likely face the Senate’s longest-serving Republican, Ted Stevens, who was indicted last month.

. Colorado. Five-term Democratic Rep. Mark Udall faces former Republican Rep. Bob Schaffer to succeed retiring Sen. Wayne Allard.

. New Hampshire. First-term Republican Sen. John E. Sununu is in a rematch against former Democratic Gov. Jeanne Shaheen.

. New Mexico. Third-term Republican Rep. Steve Pearce trails fifth-term Democratic Rep. Tom Udall in race to succeed retiring GOP Sen. Pete V. Domenici.

. Virginia. Former GOP Gov. Jim Gilmore lags behind former Democratic Gov. Mark Warner to succeed retiring Sen. John W. Warner.

Vulnerable GOP incumbents:

. Maine. Sen. Susan M. Collins, has an edge over sixth-term Democratic Rep. Tom Allen.

. Minnesota. Sen. Norm Coleman leads the Democrat, comedian-turned-candidate Al Franken.

. Mississippi. Appointed Republican Sen. Roger Wicker is up against former Gov. Ronnie Musgrove in a special election to serve out the remaining four years of retired Sen. Trent Lott’s term.

. North Carolina. Republican Sen. Elizabeth Dole is trying to hold off a challenge from Democratic state Sen. Kay Hagan.

. Oregon. Second-term moderate Sen. Gordon Smith is challenged by Statehouse Democratic Speaker Jeff Merkley.

Vulnerable Democrat:

. Louisiana. Democratic Sen. Mary L. Landrieu is in a close contest with Republican State Treasurer John Kennedy.