Mainers will head to the polls Tuesday to cast the final batch of ballots in a U.S. Senate race with national implications and to choose which presidential candidate – or candidates – will get the state’s four Electoral College votes.

Roughly 500,000 Maine residents already had voted by absentee ballot as of Monday, representing roughly half of the state’s total registered voters and three-quarters of those who typically participate in presidential elections. The flood of absentee ballots will reduce crowding at the polls on Tuesday, although physical distancing and other precautions due to the coronavirus will likely still lead to lines in some locations.

The presidential contest and the heated Senate race between Republican Sen. Susan Collins and Democrat Sara Gideon have dominated the conversation – and airwaves – in Maine for months. But voters also will cast ballots to fill the two U.S. House seats, as well as all 186 positions in the state Legislature, where Democrats and Republicans are battling for control of the Maine Senate.

Polls open at 7 a.m. and close at 8 p.m. in most locations, although hours may vary. In Maine, voters can register at the polls on Election Day and cast ballots in the races.

State health and elections officials are urging voters to be patient, wear a mask and give each other plenty of space (and respect) as municipal clerks and poll workers strive to safely pull off an election during a pandemic. Underscoring the need for safety precautions, the election is being held at a time when Maine – like many other states – is seeing record numbers of new COVID-19 infections and is reimposing restrictions on indoor gatherings starting later this week.

“Voting can be done safely and in-person so long as we follow these best practices,” Dr. Nirav Shah, director of the Maine Center for Disease Control and Prevention, said during a COVID-19 briefing Monday.

Gov. Janet Mills, meanwhile, urged Mainers to vote safely and be mindful of the state’s surging COVID-19 cases on Tuesday.

“I would ordinarily be looking forward to stopping in on a number of parties and celebrations across the state,” Mills said. “I’m not going to be doing that. I’ll be staying put watching things on TV, watching Facebook live presentations. … It’s part of what we are giving up because we all want people to stay safe.”

The Maine Secretary of State’s Office reported Monday afternoon that 502,882 voters have already returned their absentee ballots out of 530,709 that had been issued. A total of 499,931 of those received ballots have been accepted by clerks while 4,327 ballots were rejected.

Ballots can be rejected for a host of reasons, including voters failing to sign the back of the return envelope, signatures that don’t match the one on file with clerks, or ballots that were sent out but bounced back as undeliverable. But the most common reason listed was “spoiled by voter,” which often means the voter made a mistake and requested a clean ballot.

Registered Democrats have taken advantage of the absentee balloting process at much higher proportions, accounting for 238,682 of returned ballots (or 47 percent) followed by 127,322 ballots returned by independent or unenrolled voters, 120,139 returned by registered Republicans and 16,739 returned by Green Independent voters.

Many towns across Maine already have started processing and scanning – but not counting – the record number of absentee ballots delivered via mail, dropped off in dedicated boxes or cast in-person at municipal offices. A COVID-related executive order signed by Mills allowed municipal clerks to begin removing filled-out ballots from their envelopes and running them through the optical scanning machines starting last Tuesday.

Portland City Clerk Kathy Jones said that, as of Monday morning, the city had received 30,477 of the roughly 33,000 absentee ballots sent out this election. All of the those ballots were processed over the weekend, Jones said.

The processing was made easier by a new high-speed tabulator for the municipal ballots. Jones said the tabulator can read 5,000 to 7,000 municipal ballots an hour, whereas the old machine could read only 3,000 a day. She said the city had seven machines in service over the weekend to process the state ballots.

“We have all ballots entered into the machines, both state and city,” Jones said in an email Monday. “We are completely caught up except for what has come in today.”

State law requires ballot counting to be done in public. But that has been complicated by the fact that Portland City Hall is currently closed to the public because of the coronavirus pandemic. Jones said the city notified the respective campaigns when the city was beginning to process the absentee ballots and has allowed one representative from each group to observe the tabulation.

Eyes across the country will be focused on Maine as Collins – who has served 24 years in Congress – attempts to hold back Gideon in what has become one of the major battlefronts over which party will control the Senate next year. But the nation may have to wait a week or more to learn the results of the race. That’s because poll after poll shows neither Gideon nor Collins breaking the 50 percent threshold needed for an outright victory without triggering a ranked-choice voting tabulation involving supporters of independents Max Linn and Lisa Savage.

As of mid-October, Gideon had raised $69.5 million for her campaign while Collins had raised $27 million, according to federal campaign finance reports. Add in the more than $112 million dumped into the race by outside groups – much of it to pay for attack ads and negative mailers – and Maine’s Senate race will likely have seen more than $200 million in spending to influence the state’s roughly 1 million registered voters. That is nine to 10 times as much as was spent on the 2018 race for Maine’s 2nd Congressional District, which was the previously most expensive congressional race in Maine history.

Polls have repeatedly shown Gideon leading Collins, although sometimes by small margins that were within the margin of error.

An Emerson College poll of 611 likely voters released on Sunday showed Gideon over Collins 48 percent to 42 percent with Savage at 5 percent and Linn at 1 percent. Four percent of respondents were undecided in the race. A Colby College poll of nearly 900 likely voters that was released last week, meanwhile, found Gideon leading Collins 46.6 percent to 43.4 percent, which is just within the 3.3 percentage point margin of error. Savage and Linn were at 4.7 percent and 1.7 percent, respectively, with 3.6 percent undecided.

Over the weekend, Collins and Gideon made campaign stops throughout the state in hopes of turning out their supporters and winning over that slim margin of undecided voters. And both candidates were in Aroostook County on Monday.

Gideon began her day along Maine’s northern border by greeting mill workers at the Twin River Paper mill in Madawaska before making stops in Fort Kent, Presque Isle, Atkinson and Skowhegan. Collins, who is an Aroostook native, began her day at Katahdin Forest Products in Oakfield with later stops in Nashville Plantation, Portage Lake, Fort Kent, Madawaska (where she also talked to mill workers) and St. Agatha.

In the presidential race, Democrat Joe Biden appears safely ahead of President Trump when it comes to the statewide vote, based on a bevy of recent polls. But the Trump campaign has been campaigning hard in Maine’s 2nd Congressional District in hopes of picking up one of the state’s four Electoral College votes, as he did during his matchup with Hillary Clinton in 2016.

On Monday afternoon, Trump tried to rally voters in the state by tweeting out: “I gave Maine everything that Obama/Biden took away from it. 5000 square miles, Lobster, Fishing, ended tariffs from China and E.U. and much more. Vote Trump Maine!”

Poor punctuation notwithstanding, Trump was referring to his decision to rescind a national marine sanctuary designation by President Barack Obama for a region of the Gulf of Maine off the coast of southern New England where relatively few Maine-based fishermen work. And while his administration was successful in negotiating an end to long-standing European tariffs, China imposed (and then reduced) tariffs on Maine lobster in response to Trump tariffs on Chinese products.

Democrats responded by accusing Trump of “pandering” to the fishing industry.

“Four years ago, Trump promised the greatest trade deals in American history, but all we got was a failed trade war with China that caused Maine’s lobster industry to crater,” Lisa Roberts, executive director of the Maine Democratic Party, said in a statement. “Now, he’s trying to brush aside the extensive damage his disastrous trade policies have inflicted, but his attempts are too little, too late to clean up the mess he made. Our lobstermen and women won’t forget who’s responsible for throwing their industry overboard.”

Staff Writer Randy Billings contributed to this report.