Your guide to the U.S. midterms, and why they're crucial for Donald Trump

Attendees hold signs promoting U.S. President Donald Trump's Make America Great Again movement at a campaign rally for South Carolina Sen. Darline Graham in Myrtle Beach, S.C., on Aug. 21. (Jacquelyn Martin/The Associated Press)Social SharingThe upcoming U.S. midterm elections will determine whether Republican President Donald Trump will be free to pursue his agenda during his final two years in office or whether he'll face constraints from a Congress where the Democrats hold new power. 

Here's what you need to know about the midterms. 

The midterms are a contest for control of both the House of Representatives and the Senate, which together make up the U.S. Congress.

The party that controls the Senate holds sway over presidential appointments to cabinet, the courts and senior posts in federal agencies. The party that controls the House can launch impeachment proceedings and has the power to initiate budget bills .

If one party controls both, it has broad influence over legislation as well as the potential to thwart some presidential actions.

The Democrats are generally excited about their prospects in the midterms and the Republicans a little worried.

History gives the Republicans plenty of reason to fret. The incumbent president's party has suffered a net loss of seats in Congress in all but two midterms since the 1930s. The only exceptions were Bill Clinton (1998) and George W. Bush (2002), when each had sky-high approval ratings.

Trump's approval rating is anything but sky-high.

With slim majorities in each chamber, the Republicans have little wiggle room to lose seats and still maintain their hold on Congress.

The Democrats need to flip just four districts to take control of the House (if they hold on to all their current seats). The Democrats also need to take just four Republican-held states to win control of the Senate. But that's a tougher task than flipping the House because there are far fewer Senate seats up for election and winning each requires a statewide victory.

A person votes in a primary election in Oak Park, Mich., on Aug. 4. (Nic Antaya/The Associated Press)All 435 seats in the U.S. House of Representatives are up for grabs, as members of the House are elected to two-year terms.

Only 35 of the 100 seats in the Senate are up for election this fall. U.S. senators are elected to six-year terms.

Midterms, like the word implies, come at the two-year mark of a president's four-year term. This year, midterm election day is Nov. 3.

Many U.S. voters will choose to vote before then, either in advance polls or by mail-in ballots, a method the Trump administration is trying to restrict.

You may also have heard that people across the U.S. have been voting at various times since the spring in primaries, which are the contests to select each party's candidates. In most states, only people who are registered Republicans or Democrats can vote in the party's primaries.

Trump is not on the ballot in the midterms, but his influence on the campaign will be unavoidable.

Only one president since 1950 has headed into a midterm with an approval rating below 40 per cent: George W. Bush in 2006 (at 37 per cent), when his Republicans lost 30 seats in the House and six in the Senate.

Despite his low polling numbers, Trump is vowing to give "a lot of speeches" in states and districts where the races are tight to help Republican candidates. Some of those candidates are quietly asking the White House to keep him away, according to reporting by Politico.

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While U.S. polls differ slightly on the precise numbers, there's a strong thread of consistency suggesting the top issue of concern among voters right now is the cost of living.

Whether it's gas prices pushed up as a result of the war against Iran or the prices of consumer goods rising because of Trump's tariffs, pocketbook issues predominate the midterm polling.

Only one U.S. president since 1950 has held a polling approval rating below 40 per cent on the day of a midterm election. Trump is on track to be the second. (Mark Schiefelbein/The Associated Press)Trump, as the dominant figure in U.S. politics for the past decade, will undoubtedly be an issue, particularly for moderate Republican or swing voters who backed him in the 2024 presidential election but have soured on him since then, according to polling.

Turnout in midterms is always lower than during presidential election years, so each party will be striving for ways to motivate their base. For the Democrats, weakening Trump's power could be that driving force. For the Republicans, stoking fear about what a Democratic-controlled Congress would do to the economy is a key advertising message.

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Of the 35 Senate seats up for election this time, fewer than 10 are considered by campaign analysts to be at risk of changing hands.

The Republicans are considered most vulnerable in North Carolina and Maine, but polling suggests they also are in tough to hang on to their Senate seats in Ohio, Alaska, Iowa and even Texas, where no Democrat has won a statewide election since 1988.

The Democrats' biggest challenges to hold appear to be in the Senate races in Michigan and New Hampshire.

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More than 80 per cent of House seats are considered safe for either the Republicans or the Democrats, according to the University of Virginia Center for Politics newsletter Crystal Ball, which has forecast and analyzed every election for U.S. president, House, Senate and state governors since 2002.

That political inertia leaves only a few dozen districts across the country truly "in play," as campaign strategists describe races where the outcome is in doubt.

Crystal Ball has labelled 17 Republican-held districts as toss-ups, including three in Pennsylvania and two in Michigan. All will be races to watch because they're prime targets for the Democrats as they try to win back control of the House.

Mike Crawley is a correspondent for CBC News, based in Washington. He began his career as a newspaper reporter in B.C., spent six years as a freelance journalist in various parts of Africa, then joined the CBC in 2005. Mike reported on Ontario politics for 15 years. He was born and raised in Saint John, N.B.