DENVER (AP) — In its first presidential primary in 20 years, Colorado saw a last-minute surge of votes on Super Tuesday among Democrats and independents, who cast ballots for the first time without having to affiliate with either major party.

More than 1.3 million of Colorado's 3.4 million voters had cast ballots by mid-day, the secretary of state's office said. Unaffiliated voters — the largest voting bloc — made up nearly a third of that total. Most voted in the Democratic primary.

Denver retiree John Pape, a lifelong independent, was one of them. “I tend to be a moderate and more in the middle, and that's what I'm looking for in a candidate,” he said. “I'm really hoping it doesn't go all the way left to the Bernie Sanders side.”

Pape spoke as a steady stream of voters dropped off their ballots at a voting station at Harvey Park Recreation Center in Denver.

Democrat Noreen Petkovich, a 40-year-old nurse, voted for Sanders. She was drawn to his calls for healthcare coverage for all and public investment in education. But what really counted is Sanders' resonance among young voters, Petkovich said.

“Things need to change and youth is part of that,” she said.

The primary replaced a non-binding caucus system in Colorado as officials tried to get more voters involved in the national presidential race. A statewide winner of the popular vote could be declared Tuesday night. But the Democratic party won't start allocating its 67 delegates until Wednesday.

Colorado held presidential primaries from 1992 to 2000, then dropped the voting to save money. In 2016, voters approved reinstating primaries after complaining about the caucus system that involved thousands of precinct meetings to choose presidential candidates.

Four years ago, many Democratic voters couldn't get into their caucuses, which were filled to capacity. Republicans were angry that their precinct caucuses didn't include an unofficial vote for president.

Sanders defeated Hillary Clinton in the 2016 Democratic caucuses, and the Vermont senator has maintained an enthusiastic base in Colorado ever since. He has campaigned for Jared Polis, Colorado's first-term governor, and freshman U.S. Rep. Joe Neguse of Boulder.

Democrats, whose ranks have been trending younger and to the left, won the Colorado Statehouse, the governorship and all statewide offices in 2018. Health care and the environment topped voters' concerns and will resonate in this year's elections.

“The Trump campaign’s insistence that it can win here is lip service, frankly,” Democratic consultant Jason Bane said.

Some observers say Colorado's shift left in recent elections could ease a bit if Sanders, a self-described democratic socialist, emerges as the Democratic nominee.

The ranks of Republican and Democratic voters are nearly equal in the state that boasts a multibillion-dollar fossil fuels industry as well as a vibrant environmental movement.

The 2018 midterm election “was a referendum on Trump, practically a vote against every Republican on the ballot,” said Dick Wadhams, a veteran Republican strategist and former party chairman. “But this year is different. Trump is on the ballot. If Sanders also is on the ballot, that puts Colorado in play.”

The state's caucus system is still used for down-ballot races, including a re-election bid by Republican U.S. Sen. Cory Gardner. Candidates for Congress and state offices can seek votes at precinct caucuses on Saturday and at subsequent party assemblies, or petition their way onto primary ballots in June.

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Associated Press writer Nicholas Riccardi contributed to this report.