Twenty weeks before November’s midterm elections, Democrats and Republicans are cautiously eyeing a fluid political landscape, one that is likely to be dramatically altered by outside forces in the weeks and months ahead.

In interviews with about two dozen Democratic and Republican strategists, pollsters and political scientists, most agree on a few things: House Democrats are almost certain to pick up seats, and Senate Republicans are likely to add to their slim majority.

“We all think we have a handle on trends, but those are three really big possibilities out there that could alter a lot,” said Bruce Mehlman, a Republican lobbyist who closely tracks election data.

Public data gives at least a glimpse of the battle for control of Congress at the moment, a fight in which Democrats appear to have the upper hand.

The outsider party tends to make big gains in a president’s first midterm election, and that trend looks likely to hold: Democrats enjoy an advantage on the generic ballot of somewhere between 4 and 11 percentage points, according to surveys taken this month.

Almost half of voters, 48 percent, say they would be more likely to cast a ballot for a candidate who promises to be a check on Trump, while just 31 percent say they would be more likely to back a candidate who supports Trump most of the time, according to an NBC News/Wall Street Journal poll out last week. Voters in that survey said they were less likely to vote for a candidate who backed Trump’s immigration policies or the GOP tax-reform bill.

The out party’s gains have been even larger when a president’s approval rating is upside down. Trump’s approval rating is somewhere in the low- to mid-40s — lower than where former President Obama stood at this point in the 2010 midterms, when Republicans reclaimed control of Congress, but higher than where former President George W. Bush was in June 2006, 20 weeks before Democrats won the House.

And, perhaps most heartening for Democrats, their string of special election victories in state legislative districts and a House district in rural Pennsylvania hints at a level of enthusiasm among Democratic voters that far outpaces Republicans. In the NBC/Wall Street Journal poll, nearly two-thirds of Democrats said they were extremely interested in the midterm elections, while just 47 percent of Republicans said the same.

“The weather is good. The district-by-district dynamics are a lot more complex,” said Ian Russell, a media consultant who worked at the Democratic Congressional Campaign Committee during the GOP waves of 2010 and 2014.

But for each metric that appears tilted toward Democrats, there is an argument that would favor the GOP. While Democrats lead on the generic ballot, their lead is not what it was in February, when several polls pegged the GOP deficit at 15 or 16 points. While Trump’s approval ratings are lousy, they are strong in key states — North Dakota, West Virginia, Missouri — where Democrats are defending Senate seats, and in key exurban and rural House districts Democrats need to win. And special elections are no guarantee of success in a higher-turnout midterm election.

Republicans also point to voter optimism over the direction of the country. While more than half of voters still say the country is headed off track, almost 40 percent say the country is headed in the right direction.

“The landscape has moderated in our favor fairly significantly over the last couple of months. We’re in a highly volatile environment right now, so we think this has the potential to bounce around between now and the election,” said Matt Walter, who heads the Republican State Leadership Committee.

No reliable surveys have come out in Michigan, Wisconsin, North Dakota or Montana, four other Trump states with Democratic incumbents.

But those numbers come before either incumbents or well-stocked outside groups begin pouring millions into television advertising or turnout operations. The Senate Majority PAC, which backs Democrats, and the Democratic Senatorial Campaign Committee had a combined $64 million in the bank at the end of April; the National Republican Senatorial Committee and the Senate Leadership Fund, the biggest outside GOP group, had a combined $29 million on hand.

“One thing that has not changed is Senate Dems still face a tough political map,” Van Hollen said. “We’re in as strong a position as we could possibly hope to be, so I’m bullish.”

Republicans argue that the momentum has shifted in their direction, even if they still have a ways to go to reclaim an advantage.

“If the election were held today, we would keep the House,” predicted Corry Bliss, executive director of the Congressional Leadership Fund, the leading House GOP super PAC. “In December, we would have gotten our ass kicked.”

Geography also conspires to work against Democratic gains. The party’s most fervent voters are located in states and districts that are already blue, meaning the Democratic advantage in the generic ballot must be larger than simply even in order to translate into significant pickups.

“We’ve got a geographically-based representation system,” said Rob Griffin, associate director of research at the nonpartisan polling firm PRRI. “Given that dynamic, we’re at a point where even a five, six, seven, eight point lead might not translate into Democrats winning the House.”

Comparisons to previous midterm election cycles are imperfect, but instructive. Republicans privately fear their effort to hold the House is being made more complicated by the number of incumbents who have decided to retire, and the number of first-term members who represent potential swing districts. Thirty-seven Republican members of Congress are retiring or running for other office this year.

When Republicans won back the House in 1994, they beat a huge number of Democrats serving in their first or second terms, said Lara Brown, director of the Graduate School of Political Management at George Washington University.

“In some ways to me, this looks more like ’94 in the sense that if you look at the people who are in those lean Republican and Republican tossup categories, most of them are freshmen and sophomores,” Brown said.

To others, the election conjures comparisons to 2006 or 2010, when national anger at Bush and Obama swamped House members who were trying to focus on issues of local import.

“For several decades now, national trends have been more important than local issues for most races,” Mehlman said.

For all the uncertainty presented by what Mehlman calls the three "Ms" — Mueller, markets and madmen like North Korea’s Kim Jong Un — comparisons to 2006 or 2014 may be most apt. In 2006, the size of the Democratic wave did not become apparent until September, when then-Rep. Mark Foley (R-Fla.) resigned amid a sex scandal. In 2014, the rise of the Islamic State in Iraq and Syria, as well as an Ebola outbreak in West Africa, injected fear into an electorate that punished incumbent Democrats.

“Late deciders and late events are going to determine the magnitude of the Democratic pickup, and the range is probably between 18 to 40 seats,” Brown said.

Millions of voters remain undecided this year. A recent Quinnipiac University study found one in five independents did not know how they would vote this year, and a Reuters/Ipsos poll showed just 41 percent of independents favored one party or the other. Undecided voters rarely break toward the party in power in the final weeks of an election.

“Nobody who’s going to vote Republican is undecided,” said Colm O’Comartun, a strategist who formerly headed the Democratic Governors Association. “What we’re seeing [in a narrowing generic ballot] is Republicans coming home early.”