When The Cook Political Report recently added 21 House districts to its list of competitive races, every one of them was Republican-held. This is not mere coincidence.

With less than four months until the 2008 election, it is apparent that Republicans are again on the defensive after their disastrous 2006 defeats, which ceded congressional control to the Democrats. And the field of possible Democratic takeovers has grown consistently throughout this election cycle.

The Democrats’ ability to do so in the Senate, where their margin is now a slim 51-49,  is undoubted even by optimistic Republicans. And House Democrats have already added three seats in special elections this year.

The prevailing questions now focus not on whether the majorities will grow but by how much.

Will Democrats, for example, achieve a 60-seat filibuster-proof majority?

The terrain is tougher for House Democrats because many of the districts they are pursuing lean conservative. The party’s hopes, therefore, depend on its big financial advantage and on the GOP’s brand remaining unattractive to voters.

The Democratic Congressional Campaign Committee (DCCC) recently reserved $35 million worth of ads in 31 races around the country. For a committee that has trumpeted its recruiting throughout the cycle, such a power move puts its money where its mouth is.

It is also a reminder that Republicans will be severely outspent this year. The House GOP has languished at around $7 million in cash on hand for several months, while Senate Democrats had about $17 million more than Senate Republicans at the end of May.

Republicans still feel like they can play offense in a number of states and districts, but it’s clear Democrats will be going after many more in both chambers.

Democratic Senatorial Campaign Committee (DSCC) Chairman Charles Schumer (N.Y.) has classified seats according to whether his candidates have polled ahead. That measure indicates Senate Democrats are already situated to take perhaps half a dozen seats.

They have polled ahead for GOP-held seats in Alaska, Colorado, Mississippi, New Hampshire, New Mexico and Virginia. Rep. Tom Udall (D-N.M.) and former Virginia Gov. Mark Warner (D) increasingly look like shoo-ins in their states.

Sens. Norm Coleman (R-Minn.), Gordon Smith (R-Ore.) and Susan Collins (R-Maine) are attractive blue-state targets. Senate Minority Leader Mitch McConnell (R-Ky.) and Sens. Elizabeth Dole (R-N.C.) and John Cornyn (R-Texas) are also high-priority targets.

Races in Georgia, Idaho, Kansas and Nebraska could also shift if there is a Democratic tidal wave.

Senate GOPers are playing limited offense, with well-known recruits running only against Sens. Mary Landrieu (D-La.) and Frank Lautenberg (D-N.J.). Landrieu has long been a target, while Lautenberg’s age is the big question sure to be raised by his rival, former Rep. Dick Zimmer (R-N.J.).

In the House, Democrats are contesting races all over the country, including the South, which has been a Republican stronghold. But the stronghold was breached when the Democrats recently won special elections in very conservative districts in Louisiana and Mississippi.

Montgomery, Ala., Mayor Bobby Bright and Caddo Parish District Attorney Paul Carmouche in Louisiana are good examples of the pragmatism and success of Democratic recruiting. Both are well-known in their districts and hold conservative views that match those of their voters.

Those races, if won, would be the icing on what would be a big victory cake on election night.

Thanks to about 30 Republican retirements, upwards of a dozen GOP-held swing districts are legitimately up for grabs.

Most were among the seats where Democrats have reserved advertising space.

In only 12 of those 31 districts are Democrats playing defense.

The GOP does have attractive targets, including several seats in very conservative districts where the party lost in 2006 under a cloud of scandal. Tops among them are Reps. Nick Lampson’s (D-Texas) and Tim Mahoney’s (D-Fla.) seats.

Taking down those and other freshmen are key to any significant offense by Republicans.

Other targets include Georgia Reps. Jim Marshall (D) and John Barrow (D), as well as Rep. Dennis Moore (D-Kan.), Rep. Melissa Bean (D-Ill.), Rep. Paul Kanjorski (D-Pa.) and open seats in Alabama and Oregon. Aside from these seven seats, just about every other target is a freshman.

The three Democratic victors in elections this year are Reps. Don Cazayoux (La.), Travis Childers (Miss.) and Bill Foster (Ill.).

Childers and Foster both face rematches, while Cazayoux found out Friday that he will face both his former primary runoff opponent, Independent state Rep. Michael Jackson, and Republican state Sen. Bill Cassidy.