Democrats think they’ll take back the House this fall for the first time in 12 years, and most independent analysts agree with them. The well-respected Charlie Cook foresees “a very large tidal wave election” sweeping Republicans out of office.

But this elation could be premature. Republicans have used their majority to build a series of impregnable — and in some cases, illegal — seawalls against just the sort of “tidal wave” Cook sees coming. The power of incumbency in Washington is greater than ever, and even if the voters clearly believe “it’s time for a change,” they might not get it.

Mark Mellman, a well-connected Democratic pollster, is realistic about his party’s problems. “There’s a big anti-Republican wave building,” he told the Christian Science Monitor, “but that wave will crash up against a very stable political structure, and nobody will know till Wednesday morning (the day after the election) which is more important — the size of the wave or the stability of the structure.”

Don’t be fooled by Connecticut’s Democratic primary, where Sen. Joe Lieberman lost to a challenger, Ned Lamont, running on a “throw the bum out” platform. Lieberman was an easy target compared to the Republican House members who have created safe districts and huge war chests to insulate themselves from the wrath of the voters.

To be sure, those GOP lawmakers are running in a very hostile atmosphere. In the latest ABC/Washington Post poll, 52 percent of registered voters say they’d vote Democratic today, with 39 percent favoring the GOP. Strikingly, 30 percent of conservatives are ready to oust the Republicans.

Fifty-three percent describe themselves as “anti-incumbent,” while only 29 percent are “pro-incumbent.” That’s almost identical to the level of dissatisfaction registered in 1994 when Republicans overturned 40 years of Democratic rule in the House. In the Zogby poll, 22 percent rate the performance of the Republican-controlled Congress as excellent or good, 75 percent call it fair or poor

But with the election less than 100 days away, the Democrats face enormous problems of their own, and the first one is their posture on Iraq. As the Connecticut primary demonstrated, the party is badly split. Fifty-two percent backed Lamont, a harsh war critic, while 48 percent supported Lieberman, a defender of the president’s position. In the ABC/Post poll, only 27 percent said the Democrats have charted a “clear path” in Iraq, below even Bush’s 33 percent clarity rating.

A larger obstacle for Democrats is the “very stable political structure” described by Mellman. Incumbents have always used their power to enhance their re-election chances: sending free mail, solving constituent problems, steering federal funds back home. But the GOP has taken this incumbent protection racket to a whole new level.

Start with a concerted campaign to draw congressional districts that practically guarantee the election of incumbents in both parties. The result is a national scandal. The House was specifically designed, with small districts and two-year terms, to be the arm of government most accountable to the people. But the exact reverse has happened. The House is almost totally unaccountable for its actions.

The Cook Report lists only 36 races as truly competitive, or barely 8 percent of the total. Accordingly, 92 percent of the voters this fall will have no real chance to express their views through House elections, no matter how unhappy they are. And since Republicans hold a 15-seat edge, Democrats would have to win 26 of the 36 contested seats to gain a majority — a very hard task.

The second element of the Republican protection racket is money. Disgraced lobbyist Jack Abramoff has called the House Appropriations Committee the “favor factory,” but “money machine” might be a better label. The committee has channeled countless millions into the pockets of favored corporate interests, which have then poured part of their profits back into GOP campaign coffers.

One former committee member, Duke Cunningham, is already in jail for bribery. Several others, including Appropriations Chairman Jerry Lewis, could be indicted for their financial manipulations. But these criminal probes have not slowed down the GOP money machine.

In the race to replace Cunningham last spring, the national GOP spent $5 million to turn back a stiff Democratic challenge. At the end of June, Republican campaign committees enjoyed an $11 million edge over the Democrats and that gap is likely to grow.

American voters want a change in Washington. But the Republican seawalls could well be stronger than the Democratic tidal wave.