Arizona Democrats have seen a massive surge in early voting over the past week, bolstering predictions for a "blue wave" in Tuesday's elections.

Early ballot returns released Friday show Democrats are on track to narrow the voter-participation gap with Republicans to its lowest level in any midterm election in recent history.

That surge in Democratic participation could help the party flip close races or win contests for the U.S. Senate, secretary of state and superintendent of public instruction.

Democrats had significantly lagged Republicans when early ballot returns started coming in three weeks ago, leading some to speculate that the blue wave had crested.

But that changed over the past week as Democrats shaved the GOP's early-vote advantage to less than 8 percentage points. Republicans typically have a 12 percentage-point turnout edge in midterm elections.

Republicans still have a distinct advantage given they have about 136,000 more registered voters in Arizona, but the Democratic swell could swing some races.

Democratic wave arrived 'late'

Republican consultant Paul Bentz, of HighGround Public Affairs, said the numbers show the "blue wave is real, but it’s just a little late." He said Democrats have made up for a slow start.

"Before a tsunami hits, the water recedes from the beach," he said Friday afternoon. “We weren't seeing the water recede from the beach, and now we are."

Bentz said the swell could play a deciding factor in races where polling shows Democrats are close, given strong support from women and registered independents.

He mentioned three races in particular, where HighGround's polling shows a dead heat:

U.S. Senate, with Democrat Kyrsten Sinema and Republican Martha McSally.

Arizona secretary of state, with Democrat Katie Hobbs and Republican Steve Gaynor.

Arizona superintendent of public instruction, with Democrat Kathy Hoffman and Republican Frank Riggs.

Early returns also bode well for Democrats hoping to pick up seats in the Arizona Legislature — the data shows Democrats are competing for the turnout edge in traditionally GOP-leaning suburban areas, such as Chandler and Ahwatukee Foothills.

As of Friday, more than 1.46 million voters had cast early ballots. About 75 percent of Arizona voters typically cast an early mail-in ballot, so those returns are a strong barometer.

MORE:Early voting locations: Vote Saturday or Monday

Wednesday was the last day to return a ballot by mail. Voters who haven't returned their ballots should deliver them in person to any polling place on Election Day.

The early-vote tally thus far still gives the GOP a clear leg up: 41.8 percent are Republicans, compared with 33.9 percent for Democrats; 24 percent are unaffiliated, according to the Secretary of State's Office.

But the early-ballot count doesn't indicate who individual voters cast their ballots for, and many races could have cross-over voters.

Democrats invest in turnout

Democrats have invested heavily in Arizona this year, breaking spending records in several statewide races and fielding more candidates across the board.

Even before early returns came in, there was evidence of a Democratic swell: Voters in the August primary shattered the state's previous record for turnout in a party-nominating contest.

The state has also seen an increase in new voter registration, particularly among those in the youngest voting-age demographic.

Josselyn Berry, co-director of Progress Now Arizona, said the swell of Democrats voting comes after years of activist work to register and turnout more young voters and voters of color.

She said MiAZ, a coalition of progressive groups, has knocked on almost 1 million doors this cycle, encouraging voters to fill out their mail-in ballots and get to the polls.

“People tend to think of Arizona as a pretty solidly red state," Berry said. "But clearly, something big is happening."

MORE COVERAGE: