The decision by Delaware Attorney General Beau Biden (D) to forgo a Senate race that he was long expected to enter is the latest in a recent string of bad news for Senate Democrats, who are favored to defend their majority but likely to suffer larger losses than anticipated early in the election cycle.

Biden, the eldest son of Vice President Joseph Biden, said in a statement Monday that he had decided against entering the race for his father’s old seat — increasing the likelihood that presumptive GOP nominee Rep. Mike Castle will wrest control of the seat presently held by appointed Sen. Ted Kaufman (D).

Democrats aren’t conceding the race, and expect New Castle County Executive Chris Coons (D) will campaign as a political “outsider— against Castle, a fixture of Delaware politics for decades.

Biden announced his decision five days after Massachusetts state Sen. Scott Brown (R) won a special election to succeed the late Sen. Edward Kennedy (D) ­— a shocking upset that embarrassed the Democratic Party, ended its filibuster-proof majority in the Senate and instilled confidence in Republicans that they can compete just about anywhere.

In addition to Delaware, Republicans are also favored to capture the seat of Sen. Byron Dorgan (D-N.D.), who announced three weeks ago that he would not run for re-election. That surprising development paved the way for popular Gov. John Hoeven (R) to enter the race as an even heavier favorite than Castle in Delaware.

Polling data released this month also augur poorly for Senate Majority Leader Harry Reid (D-Nev.) and Sen. Blanche Lincoln (D-Ark.), who are trailing Republican opponents who are not well-known, and Sen. Michael Bennet (D-Colo.), who is running no better than even with his leading Republican opponent, former Lt. Gov. Jane Norton.

“We’re off to a good start for 2010,— said Brian Walsh, a spokesman for the National Republican Senatorial Committee.

The only really good news for Democrats this month was the Jan. 6 announcement by Sen. Chris Dodd (D-Conn.) that he will retire rather than wage a campaign for re-election in which he was considered the underdog. Democrats are strongly favored to hold the seat now that popular state Attorney General Richard Blumenthal is running instead of the politically damaged Dodd.

Democratic strategists have said they’ve long anticipated a challenging political environment for their party at the midpoint of President Barack Obama’s first term.

“We know that Democrats are facing a strong headwind in November, but that’s all the reason we need to be aggressive,— said Eric Schultz, a spokesman for the Democratic Senatorial Campaign Committee.

That Republicans are favored to make gains in the Senate this year isn’t all that surprising when you consider that the out-of-power party usually makes gains in midterm elections. Also, Republicans’ net loss of 14 Senate seats in the 2006 and 2008 elections reduced their ranks to their lowest level since Jimmy Carter was president three decades ago.

Still, it would not have been unreasonable to suggest early last year that Senate Democrats could emerge from the 2010 elections relatively unscathed — and perhaps even increase their majority by a seat or two.

In December 2008, when Republicans were still reeling from landslide Democratic wins at the presidential and Congressional levels, Sen. Mel Martinez (R-Fla.) said he would not seek re-election. He has since resigned. Within the span of one week in early January 2009, GOP Sens. Kit Bond (Mo.) and George Voinovich (Ohio) announced that they too would not seek re-election in their politically competitive states.

The following month, Sen. Judd Gregg (R-N.H.) announced his retirement in a state where Democrats have made ample gains in recent elections. In April, Sen. Arlen Specter changed his party affiliation from Republican to Democratic, giving his adopted party a 60-seat majority that stood for nine months, until Brown’s election last week.

It would take an enormous Republican upswing for the party to make the net gain of 10 Senate seats that it needs to win control of the chamber. Republicans have accomplished that feat just once in the past 60 years — in the 1980 landslide election led by Ronald Reagan. Democrats that year were the defending party in 23 of 33 Senate contests; in this year’s elections, they are the defending party in half of the 36 races on the November ballots.

Nonetheless, GOP Senate gains this November short of reaching a 51-seat majority could position the party to compete for a takeover attempt in 2012, when Democrats will be defending more than twice as many Senate seats as Republicans.

Schultz said Democrats this fall will be competitive in five of six states where Republican Senators are retiring — Florida, Kentucky, Missouri, New Hampshire and Ohio — as well as for the seats held by Sens. Richard Burr (N.C.) and David Vitter (La.).

He also suggested the Republican campaign to make major gains in the Senate has been hampered by intraparty rifts — including competitive primaries in Florida, Kentucky and New Hampshire. “Republicans are plagued with 12 bloodletting primaries, forcing their candidates rightward and outside the mainstream of the electorate,— Schultz said.

Walsh said his party is in a “far stronger position— than it was at the time of Specter’s party switch nine months ago, but he cautioned that recent good fortunes for Republicans are a reminder that a lot can change in the nine-plus months that remain until the elections.

“Our side can’t take anything for granted. We need to keep communicating our message to voters in all the key battleground states, continue to raise money and prepare for competitive elections in November,— Walsh said. “But certainly when you look at where we are now versus where we were in April of last year, it’s remarkable the progress that has been made.—