WASHINGTON — Whoever wins the presidency this November, it's all but a slam dunk they'll be working with a Democratic Congress. And it probably will be a stronger Democratic majority with more votes than it has today.

Even normally optimistic Republicans conceded last week that the landscape is stacked against them after losing their third special House of Representatives election in a row, all in what had been safe Republican districts.

"A large segment of the American public doesn't have confidence in the Republican Party," said Rep. Tom Cole, R-Okla., the party's chief political operative for House races.

"It should be a really good Democratic year in both chambers," said Stuart Rothenberg, editor of the Rothenberg Political Report. He's one of the three most authoritative nonpartisan voices on congressional races, along with Charles Cook of the Cook Political Report and Larry Sabato, the director of the Center for Politics at the University of Virginia.

They all predict that Democrats will add to their majorities in the House by six to 20 seats and in the Senate by two to five seats.

They add that the swing could be larger, but none expect the Democrats to gain enough to be able to push legislation past a Republican filibuster in the Senate or a presidential veto in either chamber.

It's also possible that some of the Democratic gains could come with the election of moderate to conservative candidates — as happened last week in Mississippi. That would mean that a Democratic president — Illinois Sen. Barack Obama or New York Sen. Hillary Clinton — might have a hard time getting even a Democratic Congress to approve all of their proposals on such issues as health care and taxes.

"Obama can propose new programs by the dozen, but odds are the Congress won't go along with most of them," said Sabato. "There will be enough moderates in both the House and Senate to force a new president to compromise."

The prospects would be worse for Arizona Sen. John McCain, the Republican candidate.

"If it's McCain," Sabato said, "he would find his domestic policies dead on arrival. His only real influence with Congress would be in the foreign sphere."

Why the likely Democratic gains?

A confluence of forces is coming together that includes an unpopular Republican president, an unpopular war, a widespread sense that the country's on the wrong track and rising costs for food, gasoline and health care. Though Democrats have shared power since they took over Congress in the 2006 election, they have yet to share much of the blame.

"The political environment is such that voters remain pessimistic about the direction of the country and the Republican Party in general," said Cole, who serves as chairman of the National Republican Congressional Committee.

Another reason is mechanical: The Republicans have more seats to defend and less money to do it.

In the Senate, where about a third of the seats are up every two years, Republicans have to defend 23 seats and Democrats only 12.

In the House, where all 435 seats are up every two years, more Republicans have decided to retire since losing majority control two years ago. Their party now has to defend more than two dozen open seats.

Republicans also have far less money, a reversal of fortune from earlier eras. As of last week, the two Republican campaign committees for House and Senate races had raised $108 million for this two-year cycle and had $24.5 million in cash left unspent.

By comparison, the two Democratic campaign committees had raised $160 million and still had $82 million in cash on hand.

"We've never seen a situation where, as a party, Democrats simply had more money and could stretch Republicans thin," said Cook. The Democrats, he said, can force the Republicans to spend money defending otherwise safe seats and bleed them dry.

The chief political operative for House Democrats, Rep. Chris Van Hollen, D-Md., warned against expecting too much, noting that it's rare for a party to gain seats the election after a big tidal-wave election like the 30-seat pickup the Democrats enjoyed in 2006.

In fact, a party hasn't followed double-digit gains with another gain since the 1970s: The Democrats gained 43 seats in 1974 and a single seat in 1976; the Republicans gained 11 seats in 1978 and 33 more in 1980.

It's still early. Many House and Senate candidates won't be selected until primaries this summer, and most voters don't tune into congressional campaigns until much later.

Republicans hope they can still fine-tune their message — national security, low taxes, smaller federal government, energy independence — to regain their footing.

But their stronger hope might lie in McCain, an ironic fate given that many conservative Republicans don't like him.

Yet it's precisely his willingness to break with them on so many occasions that might make him the perfect leader now. Recent polls show McCain neck and neck in fall match-ups with either Democrat — but show congressional Republicans trailing well behind congressional Democrats.

"McCain will be a tremendous asset," said Cole. "He's running better than the party."

Still, it's a steep climb for the Republicans to even hold their current status in the House and Senate.

The Democrats now rule the House by a margin of 236-199. They need to gain 51 more seats to have enough to pass laws over a president's veto.

They hold the Senate by 51-49, a figure that includes two independents who vote with the Democrats. They need to gain nine more seats to have enough to pass legislation over a Republican filibuster, and 16 more to have a veto-proof majority.

The early outlook, according to the experts:

• Sabato at the University of Virginia predicts the Democrats will pick up six to 12 seats in the House and three to four in the Senate.

"It could go higher" in the Senate, he said.

However, he added that he doesn't think they can win the votes needed to break filibusters. "I don't think there's any real chance they'll get 60."

• Cook predicts the Democrats will gain 10 to 20 seats in the House. "If we're wrong, it's likely to be higher, not lower."

In the Senate, he predicts the Democrats will gain four to five seats, with an all-but-certain pickup in Virginia, possible gains in Colorado, New Hampshire and New Mexico, and less likely but still possible gains in Alaska, Maine, Minnesota and Oregon.

• Rothenberg predicts Democratic gains of eight to 12 seats in the House and two to five in the Senate. "It could be a few fewer, it could be a few more," he said. "But the Democrats have the advantage on message, on money, on candidates."