WASHINGTON—For months, companies have been quietly preparing for the possibility that Democrats would win control of Congress and launch a wave of investigations. Now they are in a full sprint.A Democratic forum in July discussed projects initiated by President Trump.After dominating Tuesday’s election, Democrats have spared no time throwing themselves into investigations of American corporations, dispatching letters telling companies they are targets and setting their deadline to produce records for Jan.

4, the first full day of the new Congress. Some of America’s largest businesses—from Amazon to Apple, Live Nation to Walmart—are among the first to feel the heat.Exit polls showed that anger about affordability and corruption were key motivations for voters in handing Democrats back both chambers of the legislature, prompting the party to be aggressive in pursuing big companies.“Usually investigations are an outgrowth of an election; this time it was the point,” said Simpson Thacher partner Rafi Prober, co-head of the firm’s congressional-investigations practice.Lawyers say that companies are acting with fresh urgency.

One lawyer recalled receiving a signed retainer from a client on Wednesday, after weeks of waiting for a response. Conflict-of-interest rules limit the clients that firms can take on, meaning that businesses that didn’t already lock in experienced congressional-investigations lawyers are racing to catch up.Some businesses started preparing months ago, hiring law firms such as Akin Gump, Arnold & Porter, Crowell & Moring and Quinn Emanuel.

They have already been mapping out likely topics of scrutiny, collecting documents, interviewing employees and even running “wargame” simulations to start auditing and cataloging their exposure, according to lawyers and executives involved in the preparations.Democrats have established a long list of interest areas, with corporate ties to President Trump and his legacy projects at the top of their list. Two days after the election, Democratic Sen.

Richard Blumenthal fired off dozens of letters, putting companies on notice that he planned to turn up the pressure.Blumenthal, who is set to chair the Senate Permanent Subcommittee on Investigations, contacted household names such as Amazon, Apple, BlackRock, Booz Allen, Coinbase, Alphabet’s Google, Microsoft, Salesforce, ServiceNow and Union Pacific, among others, as known donors to Trump’s White House ballroom project.He sent additional letters to cryptocurrency firms about their Trump family ties. And he warned all of the recipients that failure to respond on time—Jan.

4 at the latest—would leave him “no choice but to consider alternative measures to obtain compliance.”Democrats on the Senate Banking Committee have also been quick to telegraph their targets. On the eve of the election, they released a report titled “Nine Ways Trump’s Corruption Is Costing You,” naming Toyota, Walmart, Live Nation and major banks among companies that allegedly received favorable treatment from the government after contributing to Trump’s inauguration.Separately, the House Judiciary Committee has started to reach out to lawyers who were involved in an antitrust lawsuit against Live Nation, which senior officials at the Justice Department settled at Trump’s direction.

The deal was hammered out at the White House with the involvement of Live Nation’s lawyers at Sullivan & Cromwell, which represents Trump in some of his personal litigation.A Live Nation spokeswoman said the settlement terms adequately address the antitrust claims made in the government’s lawsuit. “We’re ready to walk any policymaker through how and why it was reached,” the spokeswoman said.The Democrat set to chair the judiciary committee, Jamie Raskin, has also said he is ready to subpoena law firms that struck deals with the Trump administration promising to do pro bono work for the president’s favored causes to neutralize punitive executive orders.One reason Democrats are focusing on businesses is that it could provide an easier way for investigators trying to access information about the Trump administration. Companies have taken pains not to put themselves on the wrong side of the White House but now potentially face difficult questions about turning over their correspondence with Trump officials.“It’s going to put companies between a rock and a hard place,” said James Bair, a former congressional investigator who is now a partner at Crowell & Moring.Democrats on key committees have been laying the groundwork for weeks and even months, dispatching targeted document-preservation requests to companies across the tech, energy, aerospace and healthcare sectors.

Companies have responded with caution.Jared Huffman, the most senior Democrat on the House Natural Resources Committee, in late August sent preservation requests to two dozen companies listed as corporate sponsors or partners of Freedom 250 LLC, the Trump-aligned group that organized the nation’s 250th celebrations, including the Freedom 250 Grand Prix race held in Washington, D.C. Those companies included Chevron, Exxon, Lockheed Martin, Mastercard, Oracle, United Airlines and UnitedHealth Group.Some of the companies sent a short response, acknowledging receipt, according to people familiar with the communications.

They talked informally among themselves, wondering if the committee’s jurisdictional hook—linking the Washington, D.C., race to the lawmaker’s Department of Interior oversight—was a stretch. Other companies turned over documents, one of the people said.Democratic investigators are also scrutinizing data-analysis company Palantir over contracts with the Department of Homeland Security.Big questions remain about how Democrats will handle their regained power, including how strictly congressional leaders will try to ensure that different committees stay out of each others’ lanes and stick to issues that resonate with voters.Even companies that have been “out of the spotlight” are wondering if they could be pulled into oversight hearings, given the size of the Democratic majority, said Brandon Farris, executive vice president at the Steel Manufacturers Association.When Democrats had their stint in the House majority during Trump’s first term, many within the party and in legal circles saw Congress as overextending itself with too many targets and wasting valuable time with protracted court fights, said Robert Kelner, a partner at Covington & Burling who heads the firm’s congressional-investigations group.The magnitude of Democrats’ victories also means companies “will have fewer allies,” Kelner said. “The scale means that congressional Democrats will be maximally incentivized to aggressively pursue companies in investigations.”Write to Louise Radnofsky at louise.radnofsky@wsj.com, Emily Glazer at Emily.Glazer@wsj.com, C.

Ryan Barber at ryan.barber@wsj.com and Lindsay Ellis at lindsay.ellis@wsj.com