There’s not a lot of uncertainty, at least in the macro sense, about the outcomes of next week’s election in California.

It’s fairly certain that Lt. Gov. Gavin Newsom will be elevated into the governorship and that U.S. Sen. Diane Feinstein will win re-election.

It’s also highly likely, verging on certainty, that the two highest-profile statewide ballot measures, one to repeal a new package of gas taxes and automotive fees enacted last year (Proposition 6) and another to repeal decades-old restrictions on local rent-control laws (Proposition 10), will be rejected.

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The biggest question that the election will answer is whether a blue wave of disdain for President Donald Trump will flip as many as a half-dozen Republican-held congressional seats and help Democrats recapture control of the House of Representatives.

Parties and interest groups are funneling many millions of dollars into the targeted districts, mostly in Orange County and other Southern California suburbs. And in the final days of the campaign, they are concentrating on “get out the vote” efforts, even though having a month-long period of voting by mail has greatly changed those dynamics.

Whether Republicans suffer minimal congressional damage—perhaps two seats—or Democrats gain several more will hinge largely on how many registered voters actually cast votes and who they are.

Historically, voter turnout is relatively low in what are called “off-year” elections, meaning there’s no presidential contest. Four years ago, in fact, turnout hit a record low of just 42 percent of California’s registered voters, only to climb to 75 percent two years later during the sensational duel between Trump and Democrat Hillary Clinton.

It was that election that initially sparked Democratic hopes of flipping Republican congressional districts, since Clinton won in seven of those districts, and Trump’s low approval ratings since have added more fuel.

Just 39 percent of this year’s likely state voters approve of Trump’s performance in a new poll by the Public Policy Institute of California, which also found that 55 percent of them would prefer to vote for a Democratic congressional candidate.

Although low-turnout elections generically favor Republicans, this year’s lack of seriously contested top-of-the-ticket races for governor or U.S. senator could, GOP leaders worried aloud, depress turnout among their relatively small number of registered voters.

A low GOP turnout, combined with Democrats’ anti-Trump fervor, would damage Republican prospects in targeted congressional districts. Therefore, the party helped place Proposition 6 on the ballot, hoping that the chance to vote against the state’s new gas taxes would spark Republican enthusiasm.

It’s certain that turnout will be higher than 2014’s record low 42 percent but will not approach the 75 percent level of a presidential year. So 50 percent, more or less, of California’s 19 million registered voters is a reasonable guesstimate.

However, there’s no reason to believe that Republican voting will be extraordinarily high, as GOP leaders hope. PPIC’s polling this year found that 47 percent of likely voters are Democrats and 28 percent are Republicans, both a few points higher than their parties’ registrations, while 21 percent are “no party preference” independents, somewhat less than NPP registration.

PPIC also found that strong majorities of likely voters are white, women, middle-aged or older, college-educated, above-average in income and homeowners, as well as Democratic in their political leanings, even if they are NPP registrants.

With the electoral deck stacked against them, it will be a minor miracle if Republicans avoid a congressional debacle in California this year.