WASHINGTON (Reuters) - U.S. Senate Democrats, seeking to capitalize on growing disclosures about the Trump campaign’s contacts with Russia, urged a top Republican lawmaker on Tuesday to obtain President Donald Trump’s tax returns as a matter of national security. It was the second request for Trump’s returns by Senate Democrats in as many weeks, part of a Democratic campaign to pressure Republicans in Congress into obtaining the documents that could reveal conflicts of interest posed by the president’s global business empire. Trump has defied decades of precedent by refusing to release his tax returns, saying his tax affairs were under federal audit. The Internal Revenue Service has said that is no obstacle to releasing them publicly. In a letter on Tuesday to Senate Finance Committee Chairman Orrin Hatch, two of the panel’s leading Democrats said Trump’s tax returns should be pursued in light of “critical national security implications” posed by contacts between Russian Ambassador Sergei Kislyak and Trump associates, including U.S. Attorney General Jeff Sessions. “There is no debate that the conflicts in question pose a threat to American national security and the integrity of the government of the United States, and more and more keep coming to light,” wrote Senator Ron Wyden of Oregon, the committee’s top Democrat, and Senator Debbie Stabenow of Michigan. Hatch and House Ways and Means Committee Chairman Kevin Brady rejected a request for the returns last Wednesday by Senate Democrats, saying congressional actions that target individual tax returns would constitute an abuse of authority and set a dangerous precedent. On Tuesday, a Hatch spokesman said the Utah Republican had no further comment.  The two Democratic lawmakers said national security had long been a focus of the Senate Finance Committee oversight of issues involving trade, Iran and terrorism. They also said the panel previously sought the tax returns of former Enron Corp executives, oil and gas companies and the Association of Community Organizations for Reform Now, or ACORN, as part of its oversight duties. Wyden and Stabenow said they wanted the committee to hold a closed session in which lawmakers could conduct a bipartisan review of Trump’s returns.  They added: “If committee members identify ties or relations to foreign governments within these documents, we will respectfully request the chairman and members of the committee hold a vote to make that information available to the public.” 