Outside groups backing Joe Biden have spent almost twice as much as those supporting President Donald Trump on television commercials in Pennsylvania so far this year.

Democratic political groups have spent almost $21 million on airtime through this week, compared with about $11 million by pro-Trump groups, according to the ad tracking firm Advertising Analytics.

The spending advantage may have helped Biden, the presumptive Democratic nominee, jump out to an early lead in the campaign. The former vice president has consistently led Trump by a significant margin in polls of Pennsylvania voters, though strategists in both parties expect the race to tighten in the weeks ahead.

Democratic outside groups have reserved an additional $13 million worth of airtime in the fall in Pennsylvania, while their GOP counterparts have booked just $2.6 million.

» READ MORE: Pro-Biden groups have spent twice as much on TV in Pennsylvania as pro-Trump groups

The Democratic groups American Bridge 21st Century and Priorities USA both started airing anti-Trump ads before the Democratic nominating contest was effectively decided.

“American Bridge recognized early that the path to 270 electoral votes runs through Pennsylvania,” said Kyle Morse, an American Bridge spokesperson. “We finished 2019 as the biggest spender and have continued to provide the Biden campaign with the air cover it needs to make its case to voters.

“We invested heavily in recruiting former Trump voters, amplifying their stories, and targeting the voters needed to deny Donald Trump the state’s 20 electoral votes,” Morse added. “Between now and Election Day, we will continue to hold Trump’s feet to the fire.”

» READ MORE: Trump says he might give his Republican renomination speech in Gettysburg

America First Action, the principal pro-Trump super PAC, is airing commercials featuring a self-described union member and Democrat who says he won’t vote for Biden because of his “ban on fracking.”

Biden says he would halt drilling on public land but opposes a fracking ban.

Kelly Sadler, a spokesperson for America First Action, said the group was having success targeting former Democratic voters in western Pennsylvania and in the northeastern part of the state.

“This was a message that really won us the Rust Belt,” Sadler said, describing some union members’ affinity for Trump. “It’s still resonating out there today.”

» READ MORE: Fact check: No, Biden doesn’t want to ban fracking or kill the Pennsylvania jobs it supports

The Trump campaign has spent $10.9 million in the Keystone State thus far, just ahead of Biden’s $9.9 million. The Trump campaign paused its TV advertising in Pennsylvania and other states at the end of last month, after the president replaced his campaign manager.

The campaign has since resumed advertising in battleground states like North Carolina, Florida, and Arizona that begin voting earlier than others.

The Trump campaign is set to spend $17 million on the airwaves in Pennsylvania from Labor Day through the Nov. 3 election, according to Advertising Analytics.

The Biden campaign last week said it would spend $280 million on digital and TV ads in more than a dozen states, including Pennsylvania.