WASHINGTON — Democratic House candidate Rita Hart said she was dropping the challenge to her narrow election loss in Iowa, after her effort drew increasing concern among Democrats on Capitol Hill. GOP Rep. Mariannette Miller-Meeks took office in January after the Iowa State Board of Canvassers certified her victory over Ms. Hart. She won by just six votes out of the nearly 400,000 total votes cast, following a recount of the entire district. Ms. Hart filed in late December to challenge the results under the Federal Contested Elections Act. "I am saddened that some Iowans' votes won't count through no fault of their own," Ms. Hart said in a statement announcing she was dropping the effort. She blamed a "toxic campaign of political disinformation" around her challenge, while also wishing Ms. Miller-Meeks well in the post. The House Administration Committee opted on party lines earlier in March to review the contest. It isn't unusual for candidates to appeal to the House to review a race's results under the Federal Contested Elections Act, but such efforts rarely succeed. If the process had continued, it could have resulted in a vote by the full House on the matter. Ms. Hart's challenge had quickly turned into a high-octane political fight with which Democrats, particularly those in competitive districts, were becoming increasingly uncomfortable. Some Democrats were both privately and publicly expressing concern over having to vote on the result of the state-certified election in Iowa, after having objected to earlier efforts by GOP lawmakers to overturn President Donald Trump's election defeat. During a joint session of Congress on Jan. 6, some GOP lawmakers unsuccessfully challenged results in several states that President Biden won and rioters mobbed the Capitol to protest the certification of Mr. Biden's victory. Some House Democrats recently had shared their concerns with Democratic leaders and conveyed to them that they might not have enough votes to prevail, according to lawmakers and aides. Democrats currently hold a narrow 219-211 majority and can lose no more than three votes on measures opposed by all Republicans. Ms. Hart's legal team had said that it had identified 22 valid ballots that were never counted, which could reverse the outcome in Iowa.