Susan Page | USA TODAY

WASHINGTON — A year ago, Democrats were deflated and Republicans elated by Donald Trump’s victory and GOP control of the House and Senate.

No more. The approach of the 2018 midterms has reversed their moods.

The growing prospect of a wave election next November have made Democrats optimistic that they can regain control of the House of Representatives, a step that would require flipping two dozen congressional seats. Some even speculate they could claim a majority in the Senate, although that would require the electoral wave to look more like a tsunami.

Winning a majority in the House alone would give Democrats more than bragging rights. It would empower the opposition party to launch congressional investigations, convene hearings and issue subpoenas. Democratic committee chairmen and chairwomen could pursue more muscular inquiries into everything from Russian meddling in the 2016 election to Trump administration edicts at whatever federal agency you could name.

The problem for Democrats, though, is that the election isn’t being held today, when the political landscape looks so friendly. It’s more than 10 months away -- and given the turmoil of the past year or so, neither side can be sure what cataclysms could loom that would reverse moods again.

What will shape the midterms?

Here are five things to watch.

Does the tax bill work?

An odd thing happened when Congress passed the $1.5 trillion tax bill in December: The GOP cheered. So did the Democrats.

Democrats argue that the tax bill will prove to be a political albatross for the GOP. The measure, which passed entirely with Republican votes, has the lowest level of support in public polls of any major legislation enacted in the past three decades. Democrats say that most of its benefits will go to big corporations and the wealthy.

But Republicans are gambling that the tax cuts will be a financial boon for the economy and therefore a political boon for them. Advocates argue that slashing the corporate tax rate to 21% from 35% will prompt companies to invest in new equipment, increase productivity, raise wages and create jobs. "I consider this very much a bill for the middle class and a bill for jobs," Trump said when he signed the measure.

The U.S. economy has shown considerable strength since Trump’s inauguration, with the stock market hitting record highs and the unemployment rate edging down to 4.1%. But skeptics question whether the new tax cuts are deep enough for the middle class for most Americans to notice, and whether businesses will respond in ways that reward their workers rather than their stockholders.

To remind: It was a booming economy that helped President Bill Clinton extinguish Republican hopes of gaining seats in the 1998 midterms, even in the face of the Monica Lewinsky scandal.

Speaking of scandal. . . .

The risks of Russia

Allegations surrounding Moscow’s meddling in the 2016 election, which cast a shadow over Trump’s first year in office, could erupt in a storm during his second.

Special counsel Robert Mueller already has indicted four Trump associates, including his one-time campaign manager and former White House national security adviser. Two of them struck plea deals that require them to cooperate with the investigation. Congressional panels are pursuing separate inquiries.

The question of whether Russia interfered in the U.S. election, hacking emails and spreading disinformation, is considered settled by American intelligence agencies. Still at issue is whether Team Trump colluded with Russians to do that, and whether Trump or his allies tried to obstruct the federal investigation.

Getting a clean bill of health from Mueller would fortify Trump’s argument that there was no wrongdoing by him and his allies, although it wouldn’t ameliorate fears that the Russians are likely to try the same tactics again in the next election.

But a report by the special counsel that concluded there was wrongdoing by Trump’s top aides, members of his family or even the president himself could lead to the most serious constitutional crisis in the four decades since Watergate. That potential crisis would be stoked if Trump issued presidential pardons or tried to remove Mueller.

The possibility that this would lead to a serious consideration of impeachment would increase considerably if Democrats won control of the House in November.

It’s not just a number

Midterm elections are in part a referendum on the president, and approval ratings have been barometers of just how his party would fare.

Over more than a half-century, the only president who saw his party gain seats in his first midterm election was George W. Bush in 2002, in the aftermath of the shattering Sept. 11 terror attacks. In every other first midterm, the question has been whether losses for the party in power would be modest or major. The most popular presidents saw the fewest losses, the least popular ones the most.

The three presidents with the lowest approval ratings at their first midterms -- Ronald Reagan, Bill Clinton and Barack Obama -- all suffered setbacks. Under Reagan in 1982, Republicans lost ground in the House. Under Clinton in 1994, Democrats lost control of both the House and Senate. Under Obama in 2010, they lost control of the House and seats in the Senate.

The bad news for Republicans: Trump’s approval ratings have set new records. They are lower than any predecessor since modern polling began.

Is Bannon back?

Republican control, imperiled in the House, stands on friendlier ground in the Senate. The GOP is defending just eight Senate seats, while Democrats have 26 seats up, 10 of them in states Trump won last year. Five of those Democratic seats are particularly threatened, in states Trump carried by double digits.

To win the Senate, Democrats would have to hold every one of their seats and flip two more.

But they may have some unexpected help from a Trump adviser and former chief White House strategist. Steve Bannon, the past and current leader of the disruptive Breitbart News, has vowed to back anti-establishment challengers to every sitting Republican senator except Ted Cruz of Texas. “No one is safe,” he vowed in October.

For ranking Republicans, that is an unwelcome echo of the Tea Party challenges of establishment Republicans in 2010 and 2012. Conservative favorites -- Todd Akin in Missouri, Sharron Angle in Nevada, Richard Mourdock in Indiana, Christine O’Donnell in Delaware -- managed to win partisan primaries but floundered when it came to carrying general elections. That delayed Republicans from winning control of the Senate until 2014, when GOP leader Mitch McConnell had reasserted the establishment’s clout in choosing nominees.

McConnell warned of a similar dynamic ahead when Bannon’s candidate won the GOP primary for the Alabama Senate vacancy created when Trump named Jeff Sessions as attorney general. Former state Chief Justice Roy Moore defeated the candidate backed by McConnell and Trump, appointed Sen. Luther Strange. But Democrat Doug Jones won the general election in a state Trump had carried a year earlier by 28 percentage points.

Asked if Moore’s defeat in Alabama had discredited Bannon, Senate Majority Leader Mitch McConnell told The Washington Examiner: “Those political geniuses managed to elect a Democrat in the reddest state in America. I rest my case.”

There are no signs so far that Bannon has any plans to back off, though.

Watch out for women

The most dramatic political development of the Trump presidency may have been the growing political engagement of women, especially college-educated women, spurred by opposition to him.

The morning after Trump’s Inauguration a year ago, close to a half-million people participated in the defiant Women's March on Washington, more than had turned out for the inauguration itself. Since then, in gubernatorial elections in New Jersey and Virginia, in special elections in Alabama and elsewhere, women voters have turned out in larger numbers and more decisively for Democrats than they did in 2016.

In the Virginia governor’s race, Republican Ed Gillespie carried male voters by 2 percentage points; Democrat Ralph Northam won the election by carrying female voters by 22 points. In the Alabama Senate race, Republican Roy Moore won men by 14 points; Democrat Doug Jones prevailed in an upset by winning women by 16 points.

Since Trump’s election, more than 25,000 women have contacted Emily’s List, an organization that helps fund campaigns for Democratic female candidates who support abortion rights, to get information about running for office -- from local posts to state legislatures to Congress. Not all of them will follow through, of course, but unprecedented level of interest could signal a year in which women emerge as decisive voices as both voters and candidates.

Consider this context: The total number of women who contacted Emily’s List for information about running during the entire two years before the 2016 election?