WASHINGTON — The presidential campaign of Hillary Clinton and the Democratic National Committee paid for research that was included in a dossier made public in January that contained salacious claims about connections between Donald J. Trump, his associates and Russia.

A spokesperson for a law firm said on Tuesday that it had hired Washington-based researchers last year to gather damaging information about Mr. Trump on numerous subjects — including possible ties to Russia — on behalf of the Clinton campaign and the D.N.C.

The revelation, which emerged from a letter filed in court on Tuesday, is likely to fuel new partisan attacks over federal and congressional investigations into Russia’s attempts to disrupt last year’s election and whether any of Mr. Trump’s associates assisted in the effort.

The president and his allies have argued for months that the investigations are politically motivated. They have challenged the information contained in the dossier, which was compiled by a former British spy who had been contracted by the Washington research firm Fusion GPS.