Michigan is the central focus of the 2016 presidential campaign today, as state voters cast their ballots in what both Democrats and Republicans see as a critical primary.

Polls are open from 7 a.m. to 8 p.m.

Although Hawaii, Idaho and Mississippi also are voting today in the presidential races, Michigan is the single biggest primary between the Super Tuesday contests held March 1 and the March 15 elections in Ohio and Florida.

In addition to the presidential primary, Michigan voters in three state House districts are filling vacancies in the Legislature, and local communities also may be holding a special election.

Here are things that voters should know about today's election.

1. Voters must choose either a Republican or Democratic ballot.

It's an open primary, which means you do not have to be a registered Democratic or Republican to vote. However, you can only participate in the primary for one party.

In addition, election officials track which ballot that individual voters choose and that information is available to the public. So while your vote is secret, the fact of whether you participated in the GOP or Democratic primary is not.

If your community is holding a special election in addition to the presidential primary, you'll have the option of obtaining a ballot just for that election and not the presidential primary.

2. There are 59 delegates at stake in the GOP primary.

The delegates will be allocated proportionally between candidates that get at least 15 percent of the votes. If only one candidate clears the 15 percent threshold statewide, then that candidate would take all 59 delegates.

Although 13 candidates are listed on the GOP ballot, there are only four still in the race: Donald Trump, U.S. Sens. Ted Cruz and Marco Rubio and Ohio Gov. John Kasich.

Also on the ballot are nine candidates that have dropped out: Jeb Bush, Ben Carson, Chris Christie, Carly Fiorina, Lindsey Graham, Mike Huckabee, George Pataki, Rand Paul and Rick Santorum.

GOP candidates need 1,237 delegates to win the nomination. So far, Trump has 384; Cruz, 300; Rubio, 151, and Kasich has 37.

3. There 130 delegates at stake in today's Democratic primary.

Hillary Clinton and Bernie Sanders are candidates dominating the Democratic race, but today's ballot also includes Martin O'Malley, who has dropped out, and Roque "Rocky" De La Fuente.

Nationwide, the Democratic Party has 4,763 delegates in play this election cycle, with 2,382 needed to win the nomination. So far, Clinton has 1,130 and Sanders has 499.

Michigan controls 147 delegates, and 130 will be allocated proportionally based on today's primary among candidates that get at least 15 percent of the vote.

The other 17 delegates are "superdelegates," who are unpledged party leaders and elected officials who can decide for themselves whom to support.

4. Voters will be filling vacancies created by the departure of state House members Todd Courser, Cindy Gamrat and Brandon Dillon.

Voters in Allegan and Lapeer counties will be filling vacancies created in September by the ouster of Todd Courser, R-Lapeer, and Cindy Gamrat, R-Plainwell, who were forced out of the state House because of their cover up of their extramarital affair.

In the 75th State House district in Grand Rapids, voters will fill the seat created by the resignation of Brandon Dillon, who resigned last summer to become the Michigan Democratic Party chairman.

In the 80th District, which was the seat held by Gamrat, Republican Mary Whiteford, a financial planner who is vice chairperson of the Allegan County GOP, is competing against Democrat David Gernant of Plainwell and Libertarian Arnie Davidsons of Martin.

In the 82nd District, which was the seat held by Courser, Republican Gary Howell, chairman of the Lapeer County Road Commission and president of the Lapeer County Intermediate School District, is facing Democrat Margaret Guerrero DeLuca.

Both districts are strongly Republican.

In the 75th District, Democrat David LaGrand, a lawyer and business owner, faces Republican Blake Edmonds. That district tilts strongly Democratic.

5. The Michigan Secretary of State website can provide the location of your specific polling place.

A searchable database at www.michigan.gov/vote allows you to type in your name and birth date, and see a copy of the ballot at your precinct.

The website also lets voters know the location of their precinct, and even provides a Google map.

Michigan requires voters to present a photo ID to vote, although it is possible to get a waiver at the polling place. Acceptable forms of ID include a driver's license, federal or state-issued ID, U.S. passport, military ID, student or Native American Tribal ID that includes a photo. Voters without a photo ID can sign an affidavit at their polling place attesting to their identity.

6. If you haven't mailed your absentee ballot yet, you can hand deliver it.

Absentee ballots can hand-delivered to the city or township clerk's office by 4 p.m. on Election Day.

Make sure it's the clerk's office; dropping it off at a precinct won't suffice.

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Julie Mack writes for MLive/Kalamazoo Gazette. Email her at jmack1@mlive.com, call her at 269-350-0277 or follow her on Twitter @kzjuliemack.