SACRAMENTO, Calif. (Reuters) - A California neuroscientist vying to unseat Republican Congressman Dana Rohrabacher stepped up his criticism of the conservative on Tuesday, demanding he return a $1,000 contribution from indicted Trump adviser Paul Manafort.

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The Orange County seat held by Rohrabacher is key to Democrats’ hopes to increase their numbers in the U.S. House of Representatives in next year’s elections. Neuroscientist Hans Keirstead, a stem cell researcher and entrepreneur from Laguna Beach, is one of seven Democrats aiming to beat him, according to the Federal Election Commission.

“We’ve got a Russian-tainted Congressman taking Russian-tainted money from Manafort,” Keirstead said in a telephone interview on Tuesday. “Something has to be put straight here.”

Federal investigators probing Russian interference in the 2016 U.S. election on Monday charged Manafort, a lobbyist and former Trump campaign manager, with money laundering.

Rohrabacher, a senior member of the House Committee on Foreign Affairs, has frequently praised Russian President Vladimir Putin and was considered for a role in U.S. President Donald Trump’s cabinet.

His Southern California base in Orange County was for decades a Republican stronghold and a center of support for Ronald Reagan. Reliably conservative voters there have sent Rohrabacher to the House for nearly 30 years.

But the 48th Congressional district has undergone profound demographic shifts in recent years, and it is now one of nine Republican districts in California that Democrats have targeted. Hillary Clinton beat Trump in the district in 2016, winning 152,000 votes compared to his 146,600.

“These are people who care about the environment, care about social issues,” said Drew Godinich, a spokesman for the Democratic Congressional Campaign Committee in California. “Donald Trump and Donald Trump’s Republican Party are no longer a good match for them.”

Democrats’ success in the district, however, is far from assured. Despite a tilt toward Clinton for the presidency, voters there chose Rohrabacher over Democrat Suzanne Savary by more than six percentage points in 2016.

Through a spokesman, Rohrabacher indicated that he was not at this time prepared to reject Manafort - or the $1,000 donation the political operative made to his 2014 campaign.

“The Congressman advises his political opponents and the media to observe the presumption of innocence, still an American principle,” spokesman Ken Grubbs said in an email.