The Democrats are in trouble in the United States Senate come November.

That is — November 6, 2012.

On that day, 21 Democratic Senate seats will be up for reelection, plus the two Independent Senate seats that caucus with the Democrats — Joe Lieberman (CT) and Bernie Sanders (VT). Only 10 Republican seats will be up for reelection. None of the 10 Republicans had a close race in 2006 (they were the fittest that survived handily in the reddest states in what was a disastrous year for Republicans), and none are considered vulnerable (although some are getting old and could conceivably retire). But does anyone really think that Thad Cochran (the oldest Republican up for reelection in 2012) retiring in Mississippi — if he did — would change that from being a red — really red — state?

The Republicans are not going to lose a single Republican Senate seat in 2010, and — at this early stage — the same seems to be true for 2012.

This is not about the economy, the legislative agenda, the president, or even who is running for president. It is about who is up, how many are up, what their parties are, how narrowly they got in, how long (or short) a time they have been in the Senate, and how vulnerable they will be.

In fact, things are also looking very good for Republicans in 2014, when 20 Democratic seats are up in the U.S. Senate, and the Republicans only have to defend 13 seats.

The Democrats did very well in 2006 and 2008, but — of course — more wins mean more seats to defend … and many of those seats will be held by first-termers who won very narrow victories, including many who won on President Obama’s very considerable coattails in 2008. The president’s current approval rating? An all-time low of 37 percent.

Moreover, in 2012 Democratic Senators Jeff Bingaman (NM) and Dianne Feinstein (CA) will be 79, Ben Nelson (NE) will be 71, and Bill Nelson (FL) will be 70. Sanders and Lieberman will be 71 and 70, respectively.

In 2006, Lieberman won with 49.7 percent of the vote, Claire McCaskill (D-MO) won with 49.6 percent of the vote, freshman Jon Tester (D-MT) won with 49.2 percent of the vote. Freshman Jim Webb (D-VA) won by 0.6 percent. Ben Nelson (NE) has a 42 percent approval rating after his badly mismanaged support for Obamacare (he had had the highest approval rating of any senator before that). Tom Carper (D-DE) has health problems and reportedly may not seek reelection. Sherrod Brown (D-OH), Debbie Stabenow (D-MI), and Bob Casey, Jr. (D- PA) were all narrow winners in states that are now deeply troubled economically and will have had substantial Republican surges (and Democratic turnover) in 2010. Herb Kohl (D-WI) will be 67, and may face a challenge from very popular and high-profile Rep. Paul Ryan (D-WI) in a state that is suddenly and unexpectedly turning purple (just ask the soon-to-be-defeated three-term Senator Russ Feingold).

Already considered to be vulnerable Democrats in 2012 are Ben Nelson (NE) (Will he even run? Does the Democratic Party want him to?), McCaskill (MO), Webb (VA), Bill Nelson (FL), Brown (OH), Bingaman (NM), and Kent Conrad (ND). That’s seven. Add in Joe Lieberman in a possible three-way race. That’s eight.

Who are the vulnerable Republicans in 2012? Well, let’s see … there’s Scott Brown (R-MA) who only got 52 percent of the vote, but he has a 63 percent approval rating in Massachusetts and a lot of money in the bank already (and the ability to get a lot more). And — staying in New England — there is Olympia Snowe (R-ME), but she got 73 percent of the vote last time. Her strongest challenge could come from a Tea Party candidate in the primary. That’s it. Two … and they are not really vulnerable. Most likely, Republicans will keep their 10 seats in 2012, while the Democrats will struggle to defend at least eight vulnerable Senate seats.

There is West Virginia — if John Raese wins his toss-up 2010 special election for the late Robert Byrd’s seat. He would be up in 2012 and presumably would face a stiff challenge after a narrow win, but then again, if Joe Manchin (D-WV) wins, he would too.

And then there is 2014 — 20 Democrats and 13 Republicans up, and none of the 13 Republicans had tight races other than Minority Leader Mitch McConnell (R-KY) — 53 percent. He may well be Majority Leader Mitch McConnell by then — if not in January 2011, then more likely in January 2013.

Vulnerable Democrats in 2014? First-termers Kay Hagan (NC) and Mark Udall (CO) got 53 percent in the huge Democratic year of 2008; Jeanne Shaheen (NH) and Mary Landrieu (LA) got 52 percent … and there will be no Obama coattails from the sizeable number of African-American voters in Louisiana and North Carolina in 2014. Jeff Merkley (OR), Mark Begich (AK), and Al Franken (MN) won with 49, 48, and a challenged and questionable 42 percent, respectively, in 2008. And there are Democratic senators up in Arkansas (Mark Pryor), Virginia (Mark Warner), New Mexico (Tom Udall), South Dakota (Tim Johnson), and Montana (Max Baucus) who — while some of them are very popular and/or formidable campaigners, they likely will not have the advantage of being in the majority and they are in states that are not true-blue.

Moreover, in 2014, some Democratic incumbents will be getting very long in the tooth: Frank Lautenberg (NJ) — 90; Carl Levin (MI) — 80, Jay Rockefeller (WV) — 77; Tom Harkin (IA) — 74; Baucus (MT) — 72; and Dick Durbin (IL) and John Kerry (MA) — 70. Will they run? Will they run effectively?

The bottom line is that in 2012 and again in 2014 there will be many, many vulnerable Democrats in the U.S. Senate, and almost no vulnerable Republicans. If, as expected, the Republicans do not take majority control of the Senate in 2010, they certainly will set the table to do so. In 2012 they are likely to take the Senate, and in 2014, they are likely to strengthen that majority. A Republican “plus-eight” in 2010 is the consensus opinion, and it is not at all unreasonable to project Republican “plus-sixes” in 2012 and 2014 for the U.S. Senate. The result would not simply be a majority, but a filibuster-proof majority.

2018 and 2020? Well, Republicans can worry about that later, and Democrats can relish their opportunities. But the Senate tide has turned (2010), and almost certainly will continue to turn (2012 and 2014) … and it appears to be as inevitable as the ocean tides.

Update: Sen. Carper’s office denies that he has any health problems and says that he will run for re-election in 2012.

Mark A. de Bernardo, a prominent Washington, DC-area employment lawyer and lobbyist.