Secretary of State Hillary ClintonHillary Diane Rodham ClintonDem targeted by party establishment loses Texas primary Penn to Hewitt: Mueller probe born out of ‘hysteria’ Trump claims a 'spy' on his campaign tried to help 'Crooked Hillary' win MORE on Wednesday raised the stakes in a fight over Republican plans to limit U.S. funding for the United Nations, warning that the bill could jeopardize the return of U.S. troops from Iraq and Afghanistan.

"We cannot depend on United Nations missions ... to help American troops return home safely and successfully, while taking actions that will decimate the budgets that underpin those important missions," Clinton wrote to Rep. Ileana-Ros Lehtinen (R-Fla.), the chairwoman of the House Foreign Affairs Committee. "In the end, engagement through the United Nations comes at a fraction of the cost of acting alone."

Democrats have warned before that they oppose Ros-Lehtinen's bill on United Nations reform but have not until now warned the bill could threaten troop safety overseas.

Clinton's warning comes just one day before Ros-Lehtinen's committee is expected to mark up the U.N. reform bill, H.R. 2829. The bill would require the U.S. to withhold 50 percent of U.S. funding until the U.N. shifts to a voluntary funding scheme for most of its programs. Republicans say this change would bring needed accountability to the U.N., but Democrats generally argue that this change would only lessen U.S. influence in the organization.

Clinton reiterated many of those arguments in her letter.

"This bill mandates actions that would severely limit the United States' participation in the United Nations, damaging long-standing treaty commitments under the United Nations Charter and gravely harming U.S. national interests, those of our allies, and the security of Americans at home and abroad," Clinton wrote. She added that cutting funds would "undercut international collaboration in advancing core U.S. national security interests" such as stopping nuclear proliferation, combating terrorism and enforcing sanctions against certain countries.

Clinton stressed that the administration is already working to reform the U.N.: "This reform must continue," she wrote. "Withholding U.S. contributions and shifting to voluntary funding erode the concrete dividends of our leadership and undermine ongoing reform efforts at the United Nations."

She repeated her veto threat against the bill.

"For the reasons outlined above, the State Department opposes this bill," she wrote. "Should it be presented to the President, I will recommend that he veto it."

Last month, Ros-Lehtinen said the Obama administration was warning Democrats to stay away from her bill: the United Nations Transparency, Accountability and Reform Act. However, she said that opposition would not stop her from moving the bill in October.