UPDATE - California's special elections measure on Tuesday's ballot could reset balance of US Congress

ADDS EXPLANATION OF PROPOSTION 50, REVISES HEADLINE, DECK, LEDE, ADDITIONS AND EDITS THROUGHOUT

By Merve Aydogan and Michael Hernandez

HAMILTON, Canada / WASHINGTON (AA) - Polls opened Tuesday morning across California for closely watched special elections in which millions of voters will determine the fate of a key measure meant to push back on US President Donald Trump's domination of all three branches of government, especially Congress.

Gov. Gavin Newsom has put forward Proposition 50 in response to Trump's directive to Republican-led states to redraw congressional districts in order to increase the number of seats the party can flip in the US House of Representatives ahead of next year's midterm elections. Trump wants a Congress that will continue to reliably pass his legislative priorities and also ensure minimal oversight of his sweeping agenda.

Newsom's measure seeks to add five Democratic seats to California's congressional delegation to offset redistricting in Texas and so promote official resistance to Trump.

The effort comes as Trump seeks to maintain Republican control of the House during the final two years of his presidency. The party in power, in this case Republicans, typically loses seats in Congress during midterm election cycles.

Redistricting is normally done every 10 years following a census, making Trump's appeal exceedingly exceptional.

California, home to no shortage of Trump opponents, appears poised to handily pass Prop 50, the sole measure on the statewide ballot. A poll released last Thursday by the University of California at Berkeley showed that three in five likely voters support the referendum.

The finding is largely in line with other surveys that have been released in the final weeks leading up to Election Day, including a CBS News poll that found 62% of likely voters support the ballot measure.

Polling stations opened at 7 am local time (1500GMT) and will remain open until 8 pm (0400GMT Wednesday), though voters who are in line at the time of closing will be allowed to cast their ballots.

Local officials said more than 2 million ballots had already been returned in the most populous US state ahead of Election Day through drop boxes and early voting centers.

Election officials told voters that ballots sent by mail must be postmarked by Tuesday and received by Nov. 12 to be counted. Same-day voter registration is also available at county election offices and vote centers.

Initial results are expected shortly after polls close, with full certification to follow later this month.