(Updates with quotes from Nadler and Cummings)
WASHINGTON, Nov 7 (Reuters) - Armed with subpoenas and a long list of grievances, a small group of lawmakers will lead the investigations poised to make President Donald Trump's life a lot tougher now that Democrats have won a majority in the U.S. House of Representatives.
Using their control of House committees, Democrats can demand to see Trump's long-hidden tax returns, probe possible conflicts of interest from his business empire and dig into any evidence of collusion between Russia and Trump's campaign team in the 2016 election.
Trump said early on Wednesday that House investigations would be countered by investigations of Democrats by the Senate, which remains in Republican hands after Tuesday's congressional elections.
"If the Democrats think they are going to waste Taxpayer Money investigating us at the House level, then we will likewise be forced to consider investigating them for all of the leaks of Classified Information, and much else, at the Senate level. Two can play that game!" the president said on Twitter.
Senate Republican Leader Mitch McConnell's office was not immediately available for comment on Trump's tweet.
Democrats said Republican lawmakers will no longer be able to protect Trump from a watchful Congress.
"The American people have demanded accountability from their government and sent a clear message of what they want from Congress," Representative Jerrold Nadler, the New York Democrat poised to become chairman of the House Judiciary Committee, said in a tweet after Democrats claimed the majority.
Trump "may not like it, but he and his administration will be held accountable to our laws and to the American people."
Nadler, once described by Trump as "one of the most egregious hacks in contemporary politics," is one of three prominent Democrats who have clashed with the president and who will take over key House committees when the new Congress convenes in January.
The others are Elijah Cummings, who will almost certainly head the House Oversight Committee, and Adam Schiff of the Intelligence Committee, slammed by the president as "sleazy."
Control of the committees - where they are currently the highest-ranking Democrats - will give them the power to demand documents and testimony from White House officials and important figures in Trump's campaign team and businesses, and to issue subpoenas if needed.
They will also have more money and staff for investigations that could delay or derail Trump's agenda.
"I plan to shine a light on waste, fraud, and abuse in the Trump administration," Cummings said on Wednesday.
"I want to probe senior administration officials across the government who have abused their positions of power and wasted taxpayer money, as well as President Trump's decisions to act in his own financial self-interest," he said in a statement.
The White House could respond to committee demands by citing executive privilege, but that would likely result in court battles.
A first salvo in the battle is expected to come from Representative Richard Neal, who is the likely Democratic chairman of the tax-writing House Ways and Means Committee and who has said he will demand Trump's tax returns from Treasury Secretary Steven Mnuchin. Such a move could set in motion a cascade of probes into any disclosures the documents might hold.
Even before the election, Schiff said his committee would look at allegations that Russian money may have been laundered though Trump's businesses and that Moscow might have financial leverage over the president.
Nadler's panel would handle any effort to impeach Trump, depending on the outcome of Special Counsel Robert Mueller's federal probe into Russian meddling in the 2016 U.S. elections and possible Trump campaign collusion with Moscow.
The panel is expected to look for ways to protect Mueller and his probe from any Trump effort to torpedo the investigation or suppress its findings.
Trump denies any collusion by his campaign and has long denounced Mueller's investigation as a witch hunt. Moscow has denied meddling in the 2016 election.
NO RUSH TO IMPEACH
Nadler's committee is unlikely, however, to move quickly toward impeachment. He has said that any impeachment effort must be based on evidence of action to subvert the Constitution that is so overwhelming it would trouble even some Trump supporters.
Nadler, Cummings and Schiff are expected to coordinate their efforts and seek bipartisan cooperation to avoid the appearance of unbridled partisanship ahead of the 2020 presidential election.
Still, Republicans accuse Democrats of preparing to abuse their authority with political attacks on Trump and his allies. They predict a partisan drive that could backfire on Democrats, like the Republican effort to impeach former President Bill Clinton did in the 1990s.
"There will be irresistible pressure to overreach in their investigations and ultimately impeach the president," said Republican strategist Michael Steel.
Cummings' team says his Oversight Committee will also focus on public issues including skyrocketing prescription drug costs, the opioid epidemic, voting rights, the Census and the U.S. Postal Service.
(Reporting by David Morgan and Susan Cornwell Additional reporting by Patricia Zengerle, Amanda Becker, Susan Heavey and Mark Hosenball Editing by Peter Cooney and Frances Kerry)