WASHINGTON (Reuters) - President-elect Donald Trump is finding himself caught between his desire to improve relations with Russia and fellow Republicans who are pushing for a harsher response to what American spy agencies say was the Kremlin’s meddling in the U.S. presidential election. The tacit acknowledgement on Sunday by his incoming chief of staff, Reince Priebus, that Russia was behind the hacking of Democratic Party organizations suggests that Trump’s maneuvering room could be shrinking. Trump has long been dismissive of the U.S. intelligence conclusion that Russia was behind the election hacks, which Russia has denied, or was trying to help him win the November ballot, saying the intrusions could have been carried out by China or a 400-pound hacker sitting on his bed. But following a report from U.S. intelligence agencies last week blaming Russian President Vladimir Putin, Russia experts say Trump will face growing calls for a stiff military, diplomatic, economic, and perhaps also covert response after his Jan. 20 inauguration.  “The new U.S. administration will need to adopt a significantly tougher line,” said Nile Gardiner of the Heritage Foundation, a conservative think tank in Washington that is an influential voice in Trump’s transition team.     Republicans in Congress wary of Trump’s push for detente with Putin could pressure the new president to withhold the thing the Russian leader wants most: a rapid easing of the economic sanctions imposed after Russia’s 2014 annexation of Ukraine’s Crimea and its support for separatists in eastern Ukraine, the Russia experts said. U.S. intelligence agencies say that since the election, Russian spies have turned to hacking other individuals and organizations, including prominent think-tanks, in what analysts think is an effort to gain insights into future U.S. policies. Washington’s Brookings Institution, which is headed by prominent Russia expert Strobe Talbott, “received a big wave of attacks the day after the election,” but there is no reason to believe its systems have been compromised, said David Nassar, the think tank’s vice president for communications.  Republican Senator Lindsey Graham said he and fellow Republican John McCain, the chairman of the Senate Armed Services Committee, would introduce legislation with stronger sanctions than the ones now in place. “We’re going to introduce sanctions that ... will hit them in the financial sector and the energy sector, where they’re the weakest,” Graham told NBC television’s Meet the Press. Retired Marine General James Mattis, the nominee for secretary of defense who will face a Senate confirmation hearing on Thursday, is expected to advocate a stronger line against Moscow than the one Trump outlined during his election campaign.That could put him at odds with Trump’s national security adviser, retired Army Lieutenant General Michael Flynn, who has had warmer relations with Putin’s government, and with Rex Tillerson, the nominee for secretary of state, who as CEO of ExxonMobil had extensive business ties with Russia. If Mattis does push for a tougher approach to Russia, that could empower U.S. advocates for strengthening the American military presence in Europe. That could include reinforcing U.S. troops in the Baltic states and Poland, analysts say. NATO already plans to deploy 4,000 additional troops, planes, tanks and artillery to the three former Soviet republics in the Baltics and Poland this year. “There is nervousness about Trump among Europeans at NATO,” said one European diplomat. “Any grand bargain with Russia would fundamentally change NATO’s course and threaten Europe with disunity,” the diplomat said. “But we don’t expect that. NATO is seeking to reassure Baltic allies, and the United States is a big part of the deterrent.”  Some advocates of a sterner response to the Russian hacking say it should include cyber counterattacks, perhaps by leaking financial information embarrassing to some of Putin’s aides and close associates. So far, the Obama administration has refrained from such action, at least publicly, for fear that it could lead to an escalating cyberwar that could threaten critical infrastructure such as financial transactions and energy transmission. Although Trump has said the nation needed to “move on to bigger and better things” following the U.S. disclosure of alleged Russian hacking, it appears that Republican and Democratic lawmakers are unlikely to drop the issue anytime soon. McCain told NBC he wanted to create a select committee to investigate the Russian hacking, if he can convince the Republican-controlled Senate’s leaders to charge their minds.  In the meantime, he said, key Senate committees, including Armed Services and Intelligence, will investigate. Experts say the close scrutiny of Russia’s actions will come just as Trump’s administration starts to craft a comprehensive strategy on the former Cold War foe. It is likely to be weeks or longer before a clear sense of Trump’s actual Russia strategy comes into view. “Until there’s a team in place, until there’s a little more organization … I tend to think we’re not going to have clear answers,” said Heather Conley, a former deputy assistant secretary of state for Europe now at the Center for Strategic and International Studies. 