(CNN) As the 2018 election year begins, one question above all is likely to shape its outcome: Will Americans vote to constrain President Donald Trump by electing a Democrat-led Congress that will challenge and resist him, or to empower the Republicans who are increasingly working in harness with him?

Voters have increasingly viewed House and Senate elections less as a choice between individual candidates than a referendum on which party they want to control Congress -- a choice grounded in their assessments of the President. All evidence from the special elections in 2017 suggests that pattern will continue to drive voters' decisions this year.

As more voters have treated congressional elections in effect as parliamentary choices, it's grown difficult for either side to maintain the unified control of the House, the Senate and the White House that Republicans enjoy now. The last three times one party went into a midterm election holding unified control, in fact, voters have revoked it -- providing the opposition party control of one or both congressional chambers. That was the fate of Democrats under Barack Obama in 2010, Republicans under George W. Bush in 2006 and Democrats under Bill Clinton in 1994.

The ominous precedent for Republicans is that Trump's standing with the public now is weaker than each of those predecessors' was when their party lost unified control during midterm elections. (Trump's recent tweets suggesting his approval rating is as strong as Obama's are not borne out by the facts.) Depending on the survey, the share of Americans expressing approval of Trump's performance generally runs from about 35% to 40%, while the share disapproving runs from around 55% to 60%. That means he faces a "disapproval gap" of around 15 to 20 points in most surveys.

That puts Trump at the higher end of the deficits that confronted the last three presidents who lost unified control of government. Polling by the nonpartisan Pew Research Center put Obama's net approval rating at about even in October 2010, Clinton's at minus-six points in October 1994 and Bush, the closest parallel to Trump, at minus-16 in October 2006. In the elections a few weeks later, each of those president's parties suffered sweeping losses.

Attitudes toward the sitting president seeded each of these whirlwinds. In 1994, according to the Election Day exit polls, 83% of the voters who disapproved of Clinton voted Republican in House elections. In 2006, 82% of those who disapproved of Bush voted Democratic. In 2010, 84% of Obama disapprovers voted Republican.

Though Senate candidates have more opportunity than House candidates to create an independent identity (both because they receive more media coverage and spend more money on advertising), these currents have largely driven their fates as well. In 2006, Republican Senate candidates won six of the 10 races in states with exit polls where Bush's approval rating stood at 46% or higher. But Democrats won 19 of the 20 Senate races in states where Bush's approval rating reached only to 45% or less.

JUST WATCHED Santorum on Trump: He's not a complex man Replay More Videos ... MUST WATCH Santorum on Trump: He's not a complex man 10:02

Likewise in 2010, Democrats won nine of the 10 Senate races in states with exit polls where Obama's approval rating stood at 48% or more. But Republicans won 13 of the 15 Senate contests in states where Obama's approval tapped out at 47% or less.

These trends aren't absolute. In both parties, some candidates always succeed in hostile territory either because of their own skills or because the other side chooses weak opponents. Democratic Sens. Claire McCaskill of Missouri and Joe Donnelly of Indiana, for instance, won in 2012 even as their states voted against Obama because Republicans nominated deeply flawed opponents. Some House Republicans, such as Colorado's Mike Coffman or Florida's Carlos Curbelo, have shown the capacity to win districts that have voted for Democrats in two or even all three of the presidential elections since 2008. The victory of Democrat Doug Jones over Roy Moore in Alabama's US Senate race in December testified again to the limits of voters' willingness to place the party over the individual.

And yet it is far from certain that despite all of Moore's vulnerabilities, Jones could have won if Trump's position had not eroded since the previous November, even in Alabama. Trump carried over 62% of the vote in Alabama in 2016, but in last month's exit poll there, just 48% of voters said they approved of his performance, while an equal 48% disapproved. Moore held a solid 89% of those who approved of Trump but Jones carried an even larger 93% of those who disapproved.

There's no guarantee that Democrats in 2018 will routinely carry a higher percentage of voters who disapprove of Trump than Republicans do among his approvers, as Jones did. (That didn't happen in the Virginia governor's race, for instance.) The more important point is that the big special elections of 2017 sustained the past two decades' pattern of a very close relationship between voters' attitudes about the president and their choices in other elections.

Democrats still face significant structural hurdles in the 2018 election: the tendency of minority and especially younger voters to turn out in lower numbers during nonpresidential years, gerrymandered House districts in several states that fortify Republican defenses and a Senate map that forces them to defend more than twice as many seats as the GOP this year. But the backlash against Trump personally --and the majority disapproval of both the GOP's tax plan and its attempt to repeal the Affordable Care Act -- may be creating a wave that could crest above even those defenses.

Earlier in Trump's presidency, some Republican strategists had speculated that he was such a unique brand that voters unhappy with him were less likely to take that out on his party's candidates than they were for earlier presidents. But in the Virginia governor's race, 87% of voters who disapproved of Trump voted for Democrat Ralph Northam, and in New Jersey 82% backed Democrat Phil Murphy. And in both states, as in Alabama, Trump's approval rating among voters fell below his share of the vote in November 2016.

That's the same situation Republicans are facing in national polls measuring attitudes toward 2018. Trump's national job approval rating generally runs 6 to 10 percentage points below his 46 percent share of the vote in 2016. And when voters are asked which party they intend to support in next November's congressional elections, the results largely track their attitudes toward Trump.

In last month's CNN poll, 84% of Trump approvers said they intended to vote Republican for Congress, while 83% of Trump disapprovers said they intended to vote Democratic. The latest NBC/Wall Street Journal poll produced virtually identical results: 87% of Trump approvers say they will back Republicans, while 85% of Trump disapprovers say they will support Democrats. That's obviously a dangerous dynamic for Republicans when the share of voters who disapprove of Trump is so much larger than the share that approves.

This pattern presents a clear challenge for Democrats on one front: They are defending 10 Senate seats next fall in states that Trump carried in 2016. That includes several states -- West Virginia, North Dakota, Indiana, Missouri and Montana -- that Trump won by resounding margins. McCaskill in Missouri and Donnelly in Indiana top almost all handicappers' lists of the most vulnerable Senate Democrats.

But in recent elections, very few Senate incumbents from the party out of the White House have been defeated. And even in these states, where Trump is stronger than he is nationally, Democrats believe that his opponents are more energized than his supporters. That was the pattern in Virginia and Alabama, where the Republican candidates maintained preponderant advantages among Trump's best groups -- rural, evangelical and blue-collar whites -- but could not inspire them to match the surging turnout of the younger, minority and college-educated white voters hostile to him.

JUST WATCHED Sanders: GOP should worry very much about 2018 Replay More Videos ... MUST WATCH Sanders: GOP should worry very much about 2018 01:39

Veteran Democratic pollster Geoff Garin, who advised Northam in Virginia, acknowledges that Democrats in Trump-leaning states are unlikely to confront him as forcefully as those in blue areas or swing districts. But, Garin says, given the anxieties Trump has stirred during his first year, "One thing that is true in all 50 states is Democrats don't need to be afraid of Donald Trump. And that goes for the reddest of red states where Democrats are defending Senate seats. It is safe to say in every part of the country voters want to elect people who will be independent of Trump and who will stand up to him when necessary."

If Republicans have a trump card in this ominous scenario it's the possibility that growing optimism about the economy will lift the president's approval rating -- and with it the GOP's chances this fall. In a recent blog post, Kyle Clark of the Republican polling firm Public Opinion Strategies wrote, "Whether economic confidence and Trump's approval rating will continue to improve -- and whether we can expect the generic Republican vote to be the beneficiary of this increased optimism -- is a question on which congressional Republicans' fortunes could hinge."

So far, though, Trump is underperforming a typical Republican president among a group that generally is already doing well economically: college-educated white voters, who have recoiled from him on personal and cultural grounds. Trump's approval rating among college-educated whites, in fact, is now nearly as low as Obama's was among the non-college whites who fueled the GOP's 2010 landslide.

That white-collar disaffection with Trump -- along with the possibility of greater than usual midterm turnout among minority and young voters also hostile to him -- looms as the greatest threat to Republicans in November. Candidates will raise many issues next fall, but one question is likely to overshadow them all: whether Trump governs with a largely free hand through 2020 -- or sees his presidency cut in half by Democratic majorities in one or both congressional chambers.