The rest of Obama's presidency could thus become a standoff against not one, but two hostile chambers of Congress — which holds far-reaching implications, well beyond U.S. federal politics.
Americans will also be voting in hundreds of elections at the state level, including referendums on hot-button social issues like abortion, gun control and drug laws.
Some things to watch for:
—Making history: Presidential parties suffer losses in midterm elections, almost inevitably. Only three times in the last 20 midterms has the president's party gained congressional seats — under Franklin Roosevelt in 1934, Bill Clinton in 1998 and George W. Bush in 2002. But the Obama-era defeats have been particularly severe. If his Democrats lose a dozen or more House seats, Obama will have suffered the biggest losses in successive midterms of any president since the Second World War.
—Why it's so bleak: Obama's approval levels are low, but they're not historically bad at just over 40 per cent. The biggest problem for Democrats, in the current election cycle, is that the one-third of Senate seats now up for grabs happen to be disproportionately in states where Obama is especially unpopular. Key swing states include Georgia, Colorado, Louisiana, North Carolina, Kentucky, Iowa, Alaska and New Hampshire, and the latest surveys show Republicans building leads.
—The big prize: The Senate. This is where presidential appointments get approved or blocked — including nominees to the Supreme Court, members of cabinet, and ambassadors. Republicans already have the House of Representatives, and need six new seats to win the Senate. According to most polls, they're leading in nine.
—Obama's remaining power: He can still veto bills, no matter what happens Tuesday. Republicans won't get the two-thirds Senate majority they'd need to override a presidential veto, or even the 60 Senate votes they'd need to overcome some types of filibusters. Obama will still have power to issue orders to federal agencies, and he's already hinted that he intends to offer some clemency to illegal immigrants — which would produce a Republican backlash.
—A Republican Senate could: Force poison pills into legislation, and serve them to Obama. There's already talk of lacing an important budget bill with provisions to damage Obamacare, and force the president into a dilemma between saving the bill, or his cherished health law. The majority party also controls what issues come up for a vote. It also has more power to block Supreme Court nominees. One Democratic senator, Charles Schumer, last week summarized the key unspoken stakes in these elections: "Two words — Supreme Court." Half its justices are older than 75, and liberals are eager to replace them with allies who might strike down decisions on gun control, corporate political donations and restrictive voter ID laws.
—The prospects for productivity: The current Congress has been historically unproductive. A mere two per cent of bills become law, because the Democratic-controlled Senate and Republican-controlled House can't agree. One analyst found a source for optimism that things could work better if Congress was entirely controlled by one party, and the other party controlled the White House. Writing in the Washington Post, he went back four decades and showed that the most productive spells occurred under that kind of scenario. Free trade is one potential area for common ground. Republicans have already indicated that they'd like to give the president the fast-track authority he's requested to negotiate the 12-country Trans-Pacific Partnership — which is something Democrats had refused to do because they were divided on free trade.
—Canada-U.S. relations: Ottawa will be quietly rooting for several developments in the next Congress. Before delving fully into TPP trade negotiations, the Canadian government wants Obama to get fast-track authority so that Congress can't interfere with the deal once it's already been signed. Canada also hopes a new trade deal might end the threat of future Buy American bills for public procurement, by subjecting state-level contracts to the same free-trade standards as federal ones. As for the biggest Ottawa-Washington irritant, Keystone XL, Republicans are suggesting they'd make it an early priority to force a presidential decision on the long-delayed oil pipeline. Some snowbirds are also hoping to revive immigration legislation that would extend their annual maximum-stay limits in the U.S.
—State elections: These hold huge consequences for the whole country. States set election rules, and they've been known to fiddle with voter ID laws and federal district boundaries to benefit their preferred party. Republicans already control 60 per cent of state legislatures in the country, and there are predictions they could eclipse that record. Political scientist Carl Klarner, a visiting scholar at Harvard, forecasts that Democrats will lose five state Senates and nine state Houses.
—Political dynasties: Ex-president Jimmy Carter's grandson is running to follow in his footsteps as Georgia governor. Polls show Jason Carter trailing, however. George P. Bush, on the other hand, holds a commanding lead in the race for Texas land commissioner. He's the grandson of the 41st U.S. president and nephew of the 43rd.
—Referendums: Two states have already legalized marijuana and now Alaska, Oregon and Washington, D.C., will vote on similar measures. There are anti-abortion initiatives in Colorado, North Dakota and Tennessee. Amid a national trend away from strict drug sentences, California's Prop 47 would massively reduce penalties for non-violent criminals. There are competing measures on gun control in Washington State — one for tough background checks for gun-buyers, and one to restrict checks at the state level.
—The long game: Both parties describe this election as a table-setter for 2016. If the Senate map this year was difficult for Democrats, it'll be the opposite in 2016. Next time Republicans will be defending seats in states won by Obama. And they'll likely face a younger, more ethnically diverse electorate that tends to turn out in presidential election years, and tends to vote Democrat.
Charlie Cook, a renowned expert on U.S. electoral math, has summed it: "It will be Republicans in 2016 who will have the most incumbents in the crosshairs."