Americans head to the polls today in midterm elections that will determine the balance of power in Washington—and could significantly affect the future direction of the United States’ relationship with Canada.

All 435 seats in the U.S. House of Representatives are being contested, along with 35 Senate seats, including two special elections. Voters will also choose governors and other state and local representatives across the country.

Republicans currently hold narrow majorities in both chambers of Congress. Democrats need a net gain of four seats in the House and four in the Senate to take control.

The results will determine how much freedom President Donald Trump has to pursue his agenda during the remaining two years of his term.

If Democrats capture either chamber, the administration could face greater opposition, congressional investigations and increased scrutiny of its policies. A Republican victory would strengthen the president’s position and provide additional momentum for his domestic, economic and foreign-policy plans.

Affordability, immigration and healthcare have been among the campaign’s leading issues, but Canadians will be paying particularly close attention to the future of cross-border trade.

American tariffs on Canadian products have become an election issue in several closely contested states, including Maine and Michigan, where many businesses, workers and industries depend heavily on trade with Canada.

A change in congressional control could influence trade negotiations with Ottawa and lead to greater scrutiny of the president’s tariff policies. However, analysts caution that even a Democratic victory would not automatically bring immediate tariff relief because the president retains considerable authority over American trade policy.

The eventual makeup of Congress could also affect cooperation between the two countries on border security, defence, energy and other major economic issues.

Polls close at different times across the United States tonight. While some winners may be declared quickly, a complete picture of congressional control may not emerge until Wednesday or later.

Close races involving large numbers of mail ballots, recounts or legal challenges could take several days—or potentially longer—to determine.