After Tom Steyer joined the already huge field of White House hopefuls, some Democrats were instantly worried.

Their concerns -- that the millions of dollars they expected the billionaire environmental advocate to spend to help the party retake the Senate and hold the House of Representatives in next year's election will instead go into his own nascent 2020 presidential bid.

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Those concerns were documented Wednesday by The Washington Post, and gleefully amplified by Republican strategists.

"It doesn't say much for the whole Democrat field that the number one Democrat donor took a look around and decided that there's no one he can support," said Tim Murtaugh, the 2020 Trump campaign's communications director.

Steyer, though, is brushing off that narrative. "I find that funny," Steyer told Fox News on Thursday.

And he has a message for any Democrats worried about what his new campaign means for his own financial commitment to progressive causes.

"I have guaranteed that we will continue all of the grassroots efforts that we’re currently doing," he said.

Some who are concerned are pointing to Steyer's vow to spend at least $100 million of his own money on his bid for the Democratic presidential campaign. And they point to his spending $1.4 million on Wednesday alone -- just 24 hours after declaring his candidacy -- to blanket the airwaves with TV commercials.

But Steyer, in an interview with Fox News and NHTalkRadio.com, pledged "I guarantee we’re going to do all of that stuff."

The "stuff" Steyer's talking about involves the two groups he formed that turned the 62-year-old former hedge fund manager into a force in national politics.

Five years ago, he created NextGenAmerica, a grassroots advocacy organization that fueled the youth vote in 2018, helping the Democrats win back the House of Representatives. And over the past two years, he’s become one of the ringleaders in the push to impeach President Trump -- through his Need to Impeach movement.

Steyer highlighted that the petition list for Need to Impeach now stands at 8.2 million people. That's up from 6 million in January, when Steyer announced he wouldn't run for the White House after seriously mulling a presidential bid for months -- before reversing course this week.

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Republicans are crowing about Steyer's latest political move, saying it will hurt the Democrats by distracting him from his down-ballot efforts.

"The more money Steyer wastes on promoting radical policies of Democrats, the better it is for Republicans and President Trump's re-election efforts," argued Republican National Committee spokesperson Mandi Merritt. "Tom Steyer only cares about Tom Steyer."

And National Republican Senatorial Committee spokesperson Nathan Brand added that “Democrat Senate candidates won’t be able to hide from his pro-impeachment efforts and his radical job-killing, tax-hiking ‘Green New Deal’ policies.”

Even though he's no longer steering NextGenAmeirca and Need to Impeach, the organizations tell Fox News that Steyer's firmly committed to the $50 million in funding for both groups through November of next year.

"Tom’s run will not adversely impact the youth vote in 2020 because we are not scaling back our efforts. In fact, with Tom’s generous financial commitment to NextGen America for 2019-2020, we expect to run the biggest youth mobilization effort we have to date," NextGenAmerica communications director Olivia Bercow said.

She added that the group is "more committed than ever" to help Democrats take back the majority in the Senate.

A Democratic strategist and veteran of numerous Senate campaigns noted that Steyer has the ability chew gum and walk at the same time, saying that "Steyer has the money to run for president and to still be just as helpful to Democratic Senate candidates as always."

Another Democratic consultant who's steered Senate campaigns warned that "I think it would be shorted sighted on his part if he would pull back on the things that he was doing to help others to just help himself and I think it would be politically unwise and that's why I don't think he would do that."

"It would be at his political peril to pull back on the things that he has been doing for years now," the consultant added.

But a longtime, D.C.-based Democratic strategist downplayed Steyer's impact on Senate races.

The strategist emphasized that Steyer "didn't invest heavily in a lot of the critical communication campaigns that Democrats have relied on to help give us parity on television."

And he added that "a lot of his giving was inconsistent."

Fox News' Sally Persons contributed to this report.